Synthesis of Cerium Rich Intermetallics Using Molten Metal Eutectics Patricia Christine Tucker

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2012 Synthesis of Cerium Rich Intermetallics Using Molten Metal Eutectics Patricia Christine Tucker

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SYNTHESIS OF CERIUM RICH INTERMETALLICS USING MOLTEN METAL

EUTECTICS

PATRICIA CHRISTINE TUCKER

A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Degree Awarded: Fall Semester, 2012 Patricia Christine Tucker defended this dissertation on November 1, 2012. The members of the supervisory committee were:

Susan Latturner Professor Directing Dissertation

James Brooks

University Representative

Naresh Dalal Committee Member

Michael Shatruk Committee Member

The Graduate School has verified and approved the above-named committee members, and certifies that the dissertation has been approved in accordance with university requirements.

Acknowledgements

I would like to acknowledge my advisor Dr. Susan Latturner and my committee members. The support of the Departments of Chemistry, Physics, and Biology, as well as the National High Magnetic Field Laboratory (NHMFL) is greatly appreciated for allowing the use of their instrumentation. Also to undergraduate researches who provided an extra set of hands in laboratory.

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Table of Contents

List of Tables ...... vii

List of Figures ...... ix

Abstract ...... xv 1. Introduction to Metal Flux Synthesis ...... 1 1.1 Metal Flux Synthesis ...... 1 1.2 Eutectic Fluxes ...... 2 1.3 Rare Earth Carbides ...... 4 1.4 Rare Earth Borocarbides ...... 4

2. Characterization Methods ...... 6 2.1 Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) ...... 6 2.2 Transmission Electron Spectroscopy (TEM) ...... 7 2.3 X-Ray Photoelectron Spectroscopy (XPS) ...... 8 2.4 X-Ray Diffraction ...... 10 2.5 Magnetic Measurements ...... 11 2.6 Mössbauer Spectroscopy ...... 15 2.7 Differential Scanning Calorimetry (DSC)...... 16 2.8 Solid State Nuclear Magnetic Resonance (SS-NMR) ...... 16

3. Flux Growth of a New Cobalt-Zinc-Tin Ternary Phase Co7-xZn3-xSn8 and its Relationship to CoSn ...... 18 3.1 Introduction ...... 18 3.2 Materials and Methods ...... 19 3.2.1 Synthesis...... 19 3.2.2 Elemental Analysis ...... 20 3.2.3 X-ray Diffraction ...... 20 3.2.4 Magnetic Susceptibility ...... 25 3.2.5 Thermal Analysis ...... 26 3.3 Results and Discussion ...... 26 3.3.1 Co7+xZn3-xSn8 subcell and supercell crystal structures ...... 29 3.3.2 Structural relationship to CoSn ...... 31 3.3.3 Magnetic Characterization ...... 32 3.4 Conclusion ...... 33

4. Magnetic Behavior of Ce21Fe8M8C12 (M = Si, Ge, Sn, Pb) ...... 35 4.1 Introduction ...... 35

4.2 Materials and Methods ...... 36 4.2.1 Synthesis...... 36 4.2.2 Elemental Analysis ...... 37 4.2.3 X-ray Diffraction ...... 37 4.2.4 Magnetic Susceptibility ...... 40 4.3 Results and Discussion ...... 41 4.3.1 Synthesis...... 41 4.3.2 Crystal Structure ...... 42 4.3.3 Magnetic Properties...... 44 4.4 Conclusion ...... 48

5. A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare- Earth/Transition Metal Eutectic Fluxes...... 49 5.1 Introduction ...... 49 5.2 Materials and Methods ...... 50 5.2.1 Synthesis...... 50 5.2.2 Elemental Analysis ...... 51 5.2.3 X-ray Photoelectron Spectroscopy ...... 52 5.2.4 X-ray diffraction ...... 52 5.2.5 Magnetic Susceptibility ...... 57 5.2.6 Mössbauer Spectroscopy ...... 57 5.2.7 Solid State NMR Spectroscopy ...... 57 5.2.8 Resistivity Measurements ...... 57 5.3 Results and Discussion ...... 58 5.3.1 Synthesis...... 58 5.3.2 Crystal Structure of R33M14-xAlx+yB25-yC34 (R = La, Ce; M = Fe, Mn) ...... 61 5.3.3 Crystal Structure of R33M13-xAlxB18C34 (R = Ce; M = Fe, Mn) ...... 64 5.3.4 Transport Properties of Ce33Fe14-xAlx+yB25-yC34 ...... 66 5.3.5 Magnetic Behavior of R33Fe14-xAlx+yB25-yC34 Phases ...... 68 5.3.6 Magnetic Behavior of R33Fe13-xAlxB18C34 Phases ...... 72 5.4 Conclusion ...... 75

6. Intermetallics as a Catalyst for Carbon Nanotube Growth ...... 77 6.1 Introduction to Carbon Nanotubes ...... 77 6.2 New catalysts for CNT growth...... 79 6.3 Materials and Methods ...... 80 6.3.1 Synthesis...... 80 6.3.2 Scanning Electron Microscopy and Transmission Electron Microscopy ...... 81 6.3.3 X-ray photoelectron Spectroscopy ...... 81 6.3.4 Raman Spectroscopy ...... 81 6.4 Results and Discussion ...... 82 6.5 Conclusion ...... 87

7. Rare Earth Carbides and Borocarbides Grown from Ce/Co, Ce/Fe, and

Ce/Ni Eutectic Flux...... 88 7.1 Introduction ...... 88 7.2 Materials and Methods ...... 89 7.2.1 Synthesis...... 89 7.2.2 Elemental Analysis ...... 90 7.2.3 X-ray diffraction ...... 90 7.2.4 Thermal Analysis ...... 97 7.3 Results and Discussion ...... 98 7.3.1 Synthesis...... 98 7.3.2 Crystal Structure ...... 102 7.4 Conclusions ...... 109

8. Conclusions and Future Work ...... 110 8.1 A promising direction...... 110 8.2 Conclusion ...... 114

References ...... 116

Biographical Sketch ...... 136

List of Tables

1.1 A list of appropriate crucibles for the metal melt of choice...... 2

1.2 Rare-earth/transition metal compositions utilized for intermetallic synthesis ...... 3

3.1 Crystallographic parameters for the Co7+xZn3-xSn8 subcell and supercell structures ...... 21

3.2 Atomic positions for the Cmcm subcell structure of Co7.2(1)Zn2.9(1)Sn8 subcell ...... 22

3.3 Atomic positions for the Pnma supercell structure of Co6.8(4)Zn3Sn8 ...... 22

3.4 Comparison between selected bond lengths in the subcell and supercell structures of Co7+xZn3-xSn8 ...... 23

4.1 Unit cell parameters for R21T8M7C12 ...... 36

4.2 Collection parameters for all the analogues of Ce21Fe8M7C12 ...... 38

4.3 Atomic positions for analogs of La21Fe8M7C12 ...... 39

4.4 Bond lengths of La21T8Sn7C12 vs. Ce21T8Sn7C12 where T = Mn, Fe ...... 43

4.5 Bond lengths of analogs of La21Fe8M7C12 ...... 44

5.1 Single crystal collection information for selected samples ...... 53

5.2 Atomic positions and displacement parameters for select R/T/B/C phases ...... 54

5.3 Bond lengths (in Å) for R/T/B/C phases ...... 55

5.4 Unit cell parameters of all structures ...... 61

5.5 Magnetic data for R/Fe/B/C phases ...... 69

7.1 Crystallographic data collection for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, Pr31.6(5)Fe10.4(4)S6.6(4)B12C5, and Ce5(Fe/Ni)3C4 ...... 91

7.2 Crystallographic data collection for Ce4FeAlC4, Ce7Fe4C9, and Ce4FeGa0.85Al0.15C4 ...... 92

7.3 Atomic positions for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 (Z =2)...... 93

7.4 Atomic positions for Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 (Z = 2) ...... 94

7.5 Atomic positions for Ce4FeGa0.85Al0.15C4 (Z = 4) ...... 95

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7.6 Atomic positions for Ce4FeAlC4 (Z=4) ...... 95

7.7 Atomic positions for Ce7Fe2C9 (Z=2) ...... 95

7.8 Atomic positions for Ce5(Fe/Ni)3C4 ...... 96

7.9 Selected bond lengths for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, Pr31.6(5)Fe10.4(4)S6.6(4)B12C5, and Ce5(Fe/Ni)3C4 ...... 97

7.10 Selected bond lengths for Ce4FeAlC4,Ce4FeGa1-xAlxC4,and C7Fe2C9 ...... 97

7.11 Variations in reactant ratio to grow and isolate Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 in higher yield ...... 98

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List of Figures

1.1 Binary phase diagram of Ce and Co. The eutectic composition used at 76% Ce lowers the melting point of the material to 424°C, from 798°C of Ce and 1495°C of Co, which allows for centrifugation of the flux at relatively low temperatures ...... 3

3.1 Zn/Sn binary phase diagram illustrating the lowered melting point of Zn from 420°C to 199°C when combined with Sn in a 15%/85% atomic percent ratio ...... 19

3.2 Powder X-ray diffraction patterns for solid products of Co/Zn/Sn flux reactions centrifuged at 593K, with Co:Zn:Sn mmol ratios of y:1.5:8.5 (values of y indicated on figure). Calculated powder patterns of Cmcm and Pnma structures of Co7+xZn3-xSn8 are plotted, as well as the pattern of CoSn, the commonly observed product ...... 24

3.3 Powder X-ray diffraction pattern for the thermal degradation product of Co7+xZn3-xSn8 after heating to 973K under argon, and the PXRD pattern of the product of attempted sotichiometric synthesis of Co7+xZn3-xSn8. Calculated powder patterns of Cmcm and Pnma structures are plotted, as well as the pattern of byproducts Sn, CoSn, and CoSn2...... 25

3.4 Differential scanning calorimetry data for the reaction of cobalt in Zn/Sn eutectic, taken as the flux reaction was cooled from 1273K to room temperature. The inset highlights the transition from CoSn to Co7+xZn3-xSn8 ...... 27

3.5 SEM image taken of the product isolated from the reaction of cobalt in Zn/Sn eutectic when centrifuged at 823K. Protrusions on the rods are the ternary phase (elemental analysis indicates presence of Co, Zn, and Sn; the hexagonal rods themselves only contain Co and Sn) . 28

3.6 Structure of Co7+xZn3-xSn8. Unit cells are indicated by dashed lines. Cobalt atoms are represented by blue spheres, zinc atoms by red spheres, and tin atoms by grey spheres; mixed Co/Zn sites are represented by purple spheres, and half-occupied cobalt sites indicated by blue hatched spheres a) Cmcm subcell structure, viewed down the a-axis. b) Pnma supercell structure, viewed down the c-axis...... 30

3.7 Structure of Co7+xZn3-xSn8. Unit cells are indicated by dashed lines. Cobalt atoms are represented by blue spheres, zinc atoms by red spheres, and tin atoms by grey spheres; mixed Co/Zn sites are represented by purple spheres, and half-occupied cobalt sites indicated by blue hatched spheres a) Cmcm subcell structure with tin atoms removed, viewed down the b-axis. b) Pnma supercell structure with tin atoms removed, viewed down the a-axis...... 30

3.8 Comparison between the structures of CoSn and Co7+xZn3-xSn8. a) The hexagonal CoSn structure viewed down the c-axis; cobalt atoms indicated by blue spheres. b) The Cmcm structure of Co7+xZn3-xSn8, viewed down the a-axis. c) The common building block of the cobalt site (or Co/Zn site in the ternary phase) and its coordination environment...... 32

3.9 Inverse magnetic susceptibility data for the Co7+xZn3-xSn8 phase in both subcell and supercell structures...... 33

4.1 Optical microscopy image of Ce21Fe8Ge6.8Al0.2C12. It exhibits truncated cuboidal growth. The size of this crystal is approximately 3mm ...... 41

4.2 Unit cell representation of RE21T8M7C12 where green spheres represent the RE atom, blue spheres the main group element (M), red spheres represent Fe or Mn (T), and black spheres represent C atoms...... 42

4.3 Temperature dependent susceptibility measurements on crystals of Ce21Fe8Pb7C12 and Ce21Fe8Ge7C12 showed a ferromagnetic like transition at approximately 140K and 130K respectively...... 45

4.4 Temperature dependent susceptibility data for Ce21Fe8Si6AlC12, where the red squares represent the sample with just silicon added and black squres where Si and Al were directly added to the reaction ...... 46

4.5 Temperature dependent magnetic susceptibility measurements of La21Fe8Sn7C12 compared to the Ce analog. They have distantly different behavior...... 47

4.6 Temperature dependent susceptibility measurements of La21Mn8Sn7C12 compared to the Ce analog. Both compounds exhibited similar magnetic behavior where the Mn does not participate in the magnetic behavior...... 48

5.1 Powder diffraction data for products of R/Fe/B/C reactions synthesized using stoichiometric mixtures of elements in alumina crucibles, compared to the theoretical patterns for Ce33Fe13B18C34 and Ce33Fe14B25C34 (bottom two patterns) calculated from the structural parameters derived from single crystal XRD studies. Stoichiometric syntheses with cerium produce only the Ce33Fe14B25C34 phase. Attempts to make analogs of either structure with Nd or Sm produce only binary or pseudobinary phases...... 56

5.2 SEM image of crystals of Ce33Fe14B25C34 grown from Ce/Fe flux, displaying truncated octahedral growth habit ...... 59

5.3 (a) Structure of Ce33Fe13.1Al1.1B24.8C34. Red spheres are iron, green spheres are cerium, black spheres are carbon, blue spheres are boron. The boron-centered Fe14 clusters are drawn as red polyhedral and the isolated carbon anion in octahedral coordination as grey polyhedra. (b) Borocarbide-capped Fe14 cluster and associated bondlengths. (c) Ce1 coordination environment (48k site). (d) Ce2 coordination environment (12d site). (e) Ce3 coordination environment (6b site). (f) Coordination environment around the borocarbide chain ...... 63

5.4 (a) Structure of Ce33Fe13B18C34. Red spheres are iron, green spheres are cerium, black spheres are carbon, blue spheres are boron. The boron-centered Fe13 clusters are drawn as red

x polyhedral and the isolated carbon anion in octahedral coordination as grey polyhedra. (b) Fe13 cluster and associated bondlengths. (c) Ce1 coordination environment (48k site). (d) Ce2 coordination environment (12d site). (e) Ce3 coordination environment (6b site). (f) Coordination environment around the borocarbide chain ...... 66

5.5 Resistivity data for Ce33Fe13.1Al1.1B24.8C34 measured on a single crystal ...... 67

11 5.6 B MAS-NMR spectrum of Ce33Fe13.1Al1.1B24.8C34. Asterisks denote spinning side bands ...... 68

5.7 Temperature dependence of magnetic susceptibility for R33Fe14-xAlx+yB25-yC34 phases, with an applied field of 100 Oe. Data for La analog are in black; data for Ce analog (filled circles χm and open circles 1/ χm) are in red ...... 69

57 5.8 Fe Mössbauer spectra at various temperatures for 57-Fe enriched sample of Ce33Fe14- xAlx+yB25-yC34 ...... 71

5.9 Magnetization data for Ce33Fe14-xAlx+yB25-yC34 at several temperatures ...... 71

5.10 X-ray photoelectron spectra for crystals of Ce33Fe13.1Al1.1B24.8C34 after various sputtering times (bottom spectrum unsputtered; other spectra taken at 5 minute increments of sputtering at 2.5kV). Data shows the Ce 3d5/2 and 3d3/2 peaks and their satellites in positions typical for Ce3+ ...... 72

5.11 Temperature dependence of magnetic susceptibility for Ce33Fe13B18C34 at 100Oe applied field...... 73

5.12 X-ray photoelectron spectra for crystals of Ce33Fe13B18C34 after 20 minutes of sputtering. 4+ 0 The spin-orbit doublet at 900eV (3d5/2) and 918 eV (3d3/2) is characteristic of Ce (4f state). The peaks at 895 eV and 913 eV are shake-down peaks of these transitions ...... 73

5.13 Field dependence of magnetization for Ce33Fe13B18C34 at several different temperatures ...... 74

5.14 Temperature dependence of magnetic susceptibility for Pr33Fe13-xAlxB18C34, with an applied field of 100 Oe. Filled black circles χm and open blue circles 1/ χm...... 75

6.1 a) TEM image of a bundle of SWCNT b) TEM image of MWCNT ...... 78

6.2 Generally accepted method for CNT growh where a) is tip grown and b) is base grown...... 78

6.3 Structures and chemical formulas of each of the crystals studied. Isolated to layers of Fe atoms were used to see which would catalyze the growth of CNT more effectively ...... 80

6.4 SEM (left column) at TEM (right column) images of CNT growth on the surface of Ce33Fe14B25C34 crystals after reaction with methane at 690°C. a) 25 min, b) 30 min, c) 45 min, and d) 60 min of reaction time ...... 82

6.5 Raman spectra of CNT growth on Ce33Fe14B25C34 after varying amounts of reaction time with methane at 690°C, and associated TEM images ...... 83

6.6 The radial breathing mode region of Raman spectra of CNT growth on Ce33Fe14B25C34 after varying amounts of reaction time ...... 84

6.7 XPS spectra of surface growth on Ce33Fe14B25C34 after 60 min of reaction with methane at 690°C. The peaks at 284.5eV and 291.5eV after 10 min of sputtering to remove oxides indicate well-formed CNT ...... 85

6.8 SEM (left) and TEM (right) images after reaction with methane for 60 min at 690°C. a)Y5Mg5Fe4Al12Si6, showing no nanotube growth; b) Ce21Fe8Si7C12, showing growth of MWCNT...... 85

6.9 Raman spectra of Y5Mg5Fe4Al12Si6 and Ce21Fe8Si7C12 after 60 minutes of reaction with methane at 690°C. The large D-band and lack of RBM indicates that growth on Ce21Fe8Si7C12 is MWCNT; lack of distinct peaks in the spectrum for Y5Mg5Fe4Al12Si6 indicates no CNT growth ...... 86

6.10 SEM (left) and TEM (right) images of a)Y5Mg5Fe4Al12Si6 and b) Ce21Fe8Si7C12, after 60 min reaction with methane at 590°C. No reaction is observed at this temperature ...... 86

7.1 Outline of reactions attempted in Ce/Ni eutectic ...... 99

7.2 TGA/DSC analysis of a mini-eutectic flux reaction of Ce/Ni eutectic. Powder X-ray diffraction analysis of the resulting ingot showed that multiple binary phases are formed during this reaction suggesting that Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 is a small constituent of the overall reaction ...... 100

7.3 XPS data of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 showing the presence of both C and B in the sample ...... 101

7.4 Blue spheres represent Al atoms, yellow Ce atoms, red Fe atoms, black C/B atoms. Polyhedra are drawn to depict the coordination environment around the Al and Fe atoms. a) Unit cell representation of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 the a/b plane b) Al coordination by 9 Ce atoms, resulting in a square capped anti-prism c) coordination environment of trigonal planar FeC3 units that compose the zeolite-like cages...... 103

7.5 Blue spheres represent Al atoms, yellow Ce atoms, red Fe atoms, black C/B atoms. a) Ce atoms removed to show the zeolite like cages that run along the c-axis. b) Ce atoms removed to show the zeolite like structure with the bridging Fe-Fe bonds in the b/c plane. c) representation of the FeC3 cage around the square capped anti-prism ...... 103

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7.6 Blue spheres represent S atoms, green Pr atoms, red Fe atoms, black C/B atoms. Polyhedra are drawn to depict the coordination environment around the Al and Fe atoms. a) Unit cell representation of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 in the a/b plane b) Al coordination by 9 Pr atoms, resulting in a square capped anti-prism c) coordination environment of trigonal planar FeC3 units that compose the zeolite-like cages ...... 105

7.7 The black spheres represent C, the brown spheres Fe and yellow spheres Ce. This structure was synthesized in the same reaction as the Ce zeolite like structure. a)Unit cell representation of Ce5(Fe/Ni)3C4. b) Coordination environment around the terminal C atom of the trigonal planar FeC5 units. c) Coordination environment of the FeC5 unit ...... 106

7.8 Unit cell representation of Ce7Fe2C9. The black spheres represent C, the brown spheres Fe and yellow spheres Ce. Polyhedra of C and Fe atoms are shown ...... 107

7.9 Coordination environment of Fe2C8 clusters in Ce7Fe2C9. The black spheres represent C, the brown spheres Fe and yellow spheres Ce. a) Fe2C8 clusters surrounded by Ce atoms. b) Fe2C8 structural unit with Fe-Fe bond of 2.499(1)Å ...... 107

7.10 The black spheres represent C, the brown spheres Fe, blue spheres Ga/Al, and yellow spheres Ce. a) unit cell representation of Ce4FeGa1-xAlxC4 (x= 0.15,1.0) b) Ce and Ga/Al spheres removed and the Fe/C layers can be seen. c) The surrounding environment of the Fe/C chain can be seen. By adding additional unit cells, the structural units of FeC4 are observed ...... 108

8.1 Powder diffraction pattern of Ce10Co2B7C16 along with the powder diffraction patterns of those reactions containing Fe. Only peaks associated with the desired phase Ce10C2B7C16 were observed at 0.1 mmol ...... 111

8.2 Structure of Ce10Co2B7C16. Yellow atoms represent Ce, blue atoms Co, black atoms C, and grey atoms B. a) Unit cell representation. b) Two carbon atoms surrounded by 6 Ce atoms in the center of the unit cell, are represented by black polyhedral. C) Unit cell with bonding removed except those associated with C/Co cluster ...... 112

8.3 Yellow atoms represent Ce, blue atoms Co, black atoms C, and grey atoms B. a) C/Co cluster with C/Co bonds. Angles between each of the Co atoms is 90°. Each side of the Co square capped by a C atom. b) view of the Co/C unit down the c-axis with an extended view of the C/Co cluster ...... 112

8.4 Temperature dependent susceptibility measurements of Ce10Co2B7C16. Fitting of the high temperature data resulted in a calculated moment per Ce of 2.45µB. At low temperatures, a Curie tail is observed ...... 113

8.5 Field dependence data at 2K and 200K. At 2K, the moment per Ce is very low, but a small hysteresis was observed. It is possible that there are competing moments between the 10

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Ce sites and Co. At 200K, the field dependence data resulted in paramagnetic behavior which is consistent with the temperature dependent susceptibility data ...... 114

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Abstract

Metal eutectic fluxes are useful for exploratory synthesis of new intermetallic phases. In this work the use of cerium/transition metal eutectics such as: Ce/Co, Ce/Ni, and Ce/Fe have yielded many new synthetically and magnetically complex phases. Structural units that were previously observed in phases grown in La/Ni eutectic reactions have also been observed in new structures and analogs grown from cerium/transition metal eutectics. These structural units include a main group element coordinated by 9 rare-earth atoms (such as the Al@Ce9 clusters seen in Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4), trigonal planar FeC3 units (also seen in

Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4), iron clusters capped by light elements (Fe4C6 frustrated tetrahedral in Ce21Fe8M7C14, and larger Fe clusters in Ce33Fe13.1Al1.1B24.8C34). Variants of these building blocks were observed in Ce10Co2B7C16 with square Co units and chains of B and C connected to them, Fe2C8 units observed in Ce7Fe2C9, and FeC4 observed in Ce4FeGa0.85Al0.15C4 and

Ce4FeAlC4.

Two new phases were grown from Ce/Fe eutectic, Ce33Fe13.1Al1.1B24.8C34 and

Ce33Fe13B18C34 which exhibits very similar structures, but significantly different magnetic behavior. Structurally these two phases are similar. Both crystallize in the Im-3m space group, but differ by the centering of the Fe clusters. Ce33Fe13.1Al1.1B24.8C34 contains Fe clusters centered by B atoms and Al doped on the Fe2 site. In Ce33Fe13B18C34, the Fe cluster is a perfect cubeoctahedron. Ce33Fe13.1Al1.1B24.8C34 exhibits mixed valent behavior at 75K and no magnetic moment on iron, where-as Ce33Fe13B18C34 exhibits tetravalent cerium and its iron clusters undergo a ferromagnetic transition at 180K.

Another borocarbide, Ce10Co2B7C16 was synthesized from Ce/Co eutectic flux. This structure features squares of Co surrounded by chains of C and B and a sea of cerium atoms. Temperature dependent magnetic susceptibility measurements at 1 Tesla were fit to a modified

Curie-Weiss law and a moment per Ce was calculated to be 2.70µB. XPS measurements were used to confirm that Ce is in the 3+ oxidation state.

Intermetallics containing different Fe clusters (Y5Mg5Fe4AlSi, La6Fe10Al3Si, Ce21Fe8Al7- xSix, and Ce33Fe13.1Al1.1B24.8C34) were explored as potential catalysts for conversion of methane to Carbon Nanotubes (CNT). Different growth temperatures were explored. At 690ºC,

Ce33Fe13.1Al1.1B24.8C34 catalyzed the growth of single walled carbon nanotubes, Ce21Fe8Al7-xSix multiwalled carbon nanotubes, and all other structures did not catalyze the growth of CNT.

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Chapter One

Introduction to Metal Flux Synthesis

1.1 Metal Flux Synthesis Metal flux synthesis involves the use of an excess of a low melting metal as a solvent for the growth of intermetallics. Above the melting point of the flux, reactants will dissolve in the molten bath and products will crystallize. Flux synthesis is a useful synthetic technique for the growth of novel intermetallics in the form of large single crystals. This is advantageous over traditional solid state synthesis because it requires low growth temperatures. Traditional solid state synthesis requires high temperatures (in excess of 1000ºC) and has pre-determined stoichiometry of the product. The constituent elements are typically arc-welded into a button and annealed in a crucible for some length of time. The resultant products of the reaction are typically thermodynamically favored phases in powder form, and purity is checked by powder X-ray diffraction [1]. If the sample is not pure, further annealing is necessary and another powder pattern is obtained. Once it has been determined that the phase is pure, further characterization can be performed. For a metal to be considered as a solvent for flux growth, the metal must meet the following criteria: the metal must melt at low temperatures with a large difference between its melting and boiling points, the metal should be easily removed by mechanical filtration or by chemical means, and the metal should not form stable binary products with the reactants [2]. Metals that are commonly used as fluxes are Al, Bi, Ga, Pb, Sn, and Sb [3], which melt at 660°C, 271°C, 29°C, 327°C, 231°C, and 630°C respectively [4]. Once a metal has been chosen, an appropriate crucible to contain the metal must be used. Table 1.1 outlines the types of crucibles and the metals with which they can be used [5].

Table 1.1 A list of appropriate crucibles for the metal melt of choice. Metal Container Alkali metals Ta, steel Alkaline earth metals Ta, graphite for Ba, steel

Al, Ga Al2O3, MgO, BeO Mg MgO, Ta, graphite or steel

Cu, Ag, Au Graphite, MgO, Al2O3, Ta

Fe, Co, Ni Al2O3, ZrO2, ThO2

Zn, Cd, Hg Al2O3

In Al2O3, Ta Rare earths Ta, Mo, W, BeO

Bi, Sn Al2O3, SiO2, graphite

Sb SiO2, graphite

1.2 Eutectic Fluxes A eutectic is a combination of two elements that when combined in a specific ratio will melt at a lower temperature than the elements themselves and is the lowest point on the phase diagram. Eutectic fluxes are of interest because a combination of two high melting metals may form a low melting eutectic. A low melting eutectic is anything that can be easily centrifuged without having to heat the centrifuge itself. Rare-earth elements such as Ce, Pr, and Nd, are of interest for their magnetic properties when combined with a late first row transition metal such as Co, Fe, and Ni, but using these metals independently is impractical because of the high melting points of the individual elements. The R/T metal phase diagrams, where R = early rare earth and T = late first row transition metal [6], have very similar features. They have a low melting eutectic when the composition is approximately 70-80% rare earth (Table 1.2)

Table 1.2 Rare-earth/transition metal compositions utilized for intermetallic synthesis. Rare-Earth/Transition Metal % Rare-Earth Melting point °C La/Co 69 500 La/Ni 67 517 Ce/Fe 83 592 Ce/Co 76 424 Ce/Ni 78 477 Pr/Co 66 541 Pr/Ni 81 460 Pr/Fe 79 620

The most common eutectic used for exploratory synthesis in this research was the Ce/Co eutectic. When combined in a 76% to 24% atomic ratio, the melting point is 424°C. This facilitates easy centrifugation of the eutectic flux at low temperatures. Figure 1.1 depicts the Ce/Co eutectic.

Figure 1.1 Binary phase diagram of Ce and Co. The eutectic composition used at 76% Ce lowers the melting point of the material to 424°C, from 798°C of Ce and 1495°C of Co, which allows for centrifugation of the flux at relatively low temperatures [7].

1.3 Rare Earth Carbides

Rare earth intermetallic transition metal carbides of the form RxTyCz (R= rare earth, T= transition metal, C= carbon, and x,y,z denote the number of atoms) can be classified into two different categories: carbometallates and metal rich carbides. In order to distinguish between the two, a simple formula is used: (x+y)/z; where x, y, and z, equal the number of atoms in the formula unit [8]. Carbometallates are carbides that have a metal to carbon ratio of 1:2 and contain complex n- anions [TxCz] . There is no transition metal-transition metal bonding in these compounds. Carbon atoms are covalently bonded and surround the transition metal.

Examples of carbometallates include UMoC2 [9], Ho2Cr2C3 [10], and Y2ReC2 [11]. An additional example of interest is the CeNiC2 type structure. CeNiC2 shows complex magnetic behavior below 20K. As the temperature is lowered, additional anti-ferromagnetic transitions occur, because the orientation of the moments change [12]. This structure type is particularly notable because replacing the Ce with La gives rise to superconduting behavior Metal rich carbides have a metal to carbon ratio greater than 4 and have carbon atoms that occupy interstitial positions and are surrounded by both R and T atoms. These carbides are typically filled variants of binary phases [8]. An example of this type of carbide is Ln2Mn17C3-x

(Ln = La, Ce, Pr, Nd, Sm) with filled Th2Zn17 structure type. The rare earth atoms occupy the Th sites, the transition metal Mn occupies the Zn site, and the C atoms are located in the interstitial sites of the compound [13]. The R/T eutectics utilized in this research have been useful for exploring compounds that lie between carbometallates and metal rich carbides. These compounds contain anionic clusters that have both T-C and T-T bonding. Further discussion of their magnetic behavior and structural properties will be given in future chapters.

1.4 Rare Earth Borocarbides

The simplest borocarbide, B4C, is an extremely hard material. Although the stoichiometry appears to be very simple, structurally it is very complex. It contains B12 icosahedra connected in the center of the unit cell by linear C-B-C chains. Determining the exact stoichiometry is difficult; X-rays cannot distinguish between C and B. Bond length, occupancy,

4 and phase diagram analysis of this system resulted in the conclusion that the amount of carbon ranges from 4.8-9.7 mole percent, resulting in an actual stoichiometry of B10C [14]. There is a wealth of literature on rare earth borocarbides. Examples of these include

RE10B9+xC10-x(RE = Gd,Tb; x ≈ 2) [15], REBC (RE= Ce, Pr, Nd) [16], R10B9C12 (R = Ce, Pr, Nd)

[17], and R2BC (R = Nd, Pr) [18], to name a few. The main building blocks in all of these structures are chains of B and C surrounded by rare earth atoms. In Ce10B9C12, complex magnetic behavior was observed. Long range ordering was attributed to the geometric arrangement of the rare-earth atoms. In REBC, the Ce-containing analog did not follow the Curie-Weiss law. A strongly reduced moment was calculated and was attributed to strong 4f/5d and s/p hybridization involving the C and B atoms [16].

Another commonly observed borocarbide structure type is the modified ThCr2Si2 stucture type. RT2B2C where (R = Y, Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, and Yb; T = Co, Ni) crystallize in the LuNi2B2C structure and has an additional C atom in the plane of the rare-earth [19,20]. Magnetic ordering in these compounds is dependent on the R/T combination. For example,

CeNi2B2C did not exhibit superconductivity – unlike YNi2B2C – due to its mixed valent behavior. Replacing Ni with Co resulted in antiferromagnetic behavior. Exploration of new borocarbide structure types was aided by the use of R/T eutectic fluxes. The new structure types observed are either thermodynamically stabilized or kinetically stabilized phases. Physical property measurements on these compounds were complicated by their structure as well as the number of R and T metal sites in these compounds.

Chapter Two

Characterization Methods

2.1 Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) Scanning Electron Microscopy (SEM) is used to image surfaces under high magnification. Energy Dispersive Spectroscopy (EDS) is a complementary technique used to analyze the elemental composition of the sample. Characteristic X-rays are generated when the electron beam interacts with the sample and ejects a core electron causing relaxation of a higher energy electron to fill the vacancy. These are detected by the EDS and element composition can be determined. When the beam first interacts with the surface of the sample, it penetrates it by first forming a channel and then expands outward forming a pear shaped interaction area. There are two types of interactions: elastic scattering (no loss in kinetic energy) and inelastic scattering (some energy is lost in the scattering event). When backscattered, or elastically scattered electrons, escape the sample, topographical imaging can be obtained; regions that are protruding from the surface release a larger number of backscattered electrons (BSE) and appear brighter in the image. BSE intensity is also dependent on element composition, which helps to determine the homogeneity of the sample. The brighter regions contain elements that have heavier masses and the darker regions lighter elements. Secondary electrons are generated when the incoming beam ejects core electrons from the atoms. Then, an electron in a higher energy orbital can relax to fill the resulting vacancy in the lower energy orbital and characteristic X-rays are released. Electrons that are ejected from high energy orbitals by the transfer of energy from the inelastic scattering event are Auger electrons and are not important in the determination of elemental composition. X-ray generation is favored for high atomic number elements [21]. The X-rays generated from the relaxation event are detected by the EDS. The energy of the characteristic X-rays emitted is the energy difference between two electrons in different shells. The transition is defined by which shell the electron relaxes to. For example, if an electron is ejected from the K shell and the electron relaxes from the L shell, Kα X-rays are generated. For qualitative analysis, Mosley’s law is used because it defines the relationship

6 between the wavelength of characteristic X-rays (λ) and atomic number (Z). B and σ are constants that depend on the specific energy shells in the atoms (equation 2.1) [22].

(2.1) As the size of the atom increases, so does the possibility of other transitions. The common transitions that are observed are Kα, Kβ, Lα, Lβ, and Lγ. Above atomic number 60, Mα energies are observed. For some elements, there is some overlap of their characteristic X-ray energies. For example, if Fe and Co are present in the sample, it is possible for the Kβ of Fe to overlap with the Kα of Co and give a positive result for Co even if it isn’t present in the sample. Then, line-shape analysis becomes important [22]. For samples containing B and C, X-ray photoelectron spectroscopy was used to identify these elements.

2.2 Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) is similar to SEM because it uses an electron gun that interacts with the sample, and electromagnetic lenses are used to focus the electron beam. Instead of interacting with the surface of the sample, as in SEM, TEM requires a very thin sample that allows electrons to pass through it to impact the detector on the other side. An image is generated and can be recorded using a CCD camera. The electron gun and electromagnetic lenses are the same as those used in SEM. TEM samples must be thin enough to be able to transmit electrons. There are many ways to ensure electrons pass through the sample. Common techniques are electrolytic thinning where a dimple is formed that is 100nm in thickness, ion milling, which ablates atoms to thin the sample, and ultramicrotomy, which is used commonly on polymers or biological samples and sections out a small piece of sample [21]. Samples that were studied in this work did not undergo any of these methods because they were not useful to the preparation of our samples. The TEM sample holder is a metal disc with a Cu grid that has a thin layer of carbon to hold the sample in place. It was necessary to grind the crystal covered in nanomaterials into a fine powder and add methanol to form a colloidal solution. This solution was then added dropwise to a TEM grid. When the methanol evaporated, the sample remained behind containing tiny shards of crystals and nanomaterials.

2.3 X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a useful spectroscopic technique for the determination of elemental composition and oxidation state of transition metals and some rare earth elements such as Ce, Sm, and Eu. This spectroscopic technique can be used in addition to EDS analysis because it is sensitive to the presence of light elements, such as C and B, which are not easily identified by EDS. Identification of oxidation states is especially important when complex magnetic behavior is observed [23]. The XPS technique involves bombarding the sample with X-rays and measuring the kinetic energies of the ejected electrons. Analysis of the sample requires an ultra high vacuum (UHV) environment and either Al Kα or Mg Kα X-rays with energies of 1.4866Å and 1.2536Å, respectively. These energies are considered “soft” X-rays because they are much lower in energy than those required for X-ray diffraction. When low energy X-rays interact with the surface of the sample, core electrons, rather than valence electrons, are ejected from the sample. The ejected electrons are collected in a concentric hemispherical analyzer or CHA. The CHA is composed of two concentric hemispheres with radii R1 and R2 and two negative potentials V1 and V2. When a potential is applied a median equipotential surface is generated. This potential is called pass energy and can be calculated by E=eVo. The electrons that pass through the CHA are injected tangentially to the median surface. At higher pass energies, the spectra are better resolved, but important features such as take off peaks are not observed [21]. When electrons are ejected from the sample, the ejected electrons possess a certain amount of kinetic energy which contribute to the binding energy. The following equation can be used to calculate the binding energy of the core electrons, where W depends on both the material and the spectrometer, h is Plank’s constant and ν is the frequency (equation 2.4) [24]. Each element has characteristic binding energies associated with its various electron shells; these energies may be slightly affected by oxidation state. (2.4) After the binding energies of the electrons are obtained, a spectrum is plotted where the x-axis is the binding energy (BE) with units of electron volts (eV), and the y-axis is defined as intensity or arbitrary units (A.U.). A survey scan is performed first to see which predominant peaks are present in the sample. This initial scan is used to identify the presence of elements in

8 the sample. Typically binding energies are scanned from 1000eV to 0eV. In this scan, a step- like background is observed which is caused by the inelastic scattering of the electrons in the solid. The next step is to select elemental regions of interest and perform multiplex scans. Multiplex scans are small individual scans that focus on a specific region of interest. More detail and precise locations of the peaks can be obtained through multiplex scans For all samples studied, one of the problems is the presence of oxygen on the surface of the sample. Even though UHV is utilized, trace amounts of oxygen may still be present in the form of oxide coating on the sample or physisorbed into the carbon tape. One way to remove oxygen is by sputtering. High intensity Ar+ ions (1keVà3keV) bombard the surface of the sample as a way to clean the surface. Typically samples were exposed to Ar+ ions for five- minute intervals. In-between these five-minute intervals, multiplex scans were performed to monitor the change in peak height and shape. Oxygen was included in multiplexes as a way to monitor whether or not surface oxidation was removed. Once this peak was no longer present, the surface of the sample was deemed to be clean and further analysis could be performed. A potential problem associated with sputtering is the generation of oxide species on the surface. Heat generated by sputtering can react with elements present in the sample and provide the energy necessary for O to form oxides with these elements [21]. Accurate identification of the peaks present in the sample requires a well calibrated system. The position of the C 1s peak at 285 eV is a useful calibration standard because its position is not easily shifted by the surrounding chemical environment. If this peak is not accurate, a correction factor can be added to the entire spectrum. Once the spectrum is corrected, the other peaks maybe accurately identified, and any real chemical shifts in the binding energies can be identified. Chemical shifts of binding energies are caused by the chemical environment surrounding the element of interest. Ideal values for the position of these peaks can be thought of as a starting point because the electronic properties of the sample can play a crucial role in understanding the shifts of these peaks from their ideal location. If a sample electrically contacts with the spectrophotometer, W does not change; however, if an insulating sample is being studied, W is unknown and is related to the surface potential of the sample [24].

2.4 X-ray Diffraction Technological advances have made X-ray diffraction a key characterization method in solid state and organometallic chemistry. This technique allows structure determination of crystalline materials, providing detailed information about atom positions and bondlengths in a solid. Layers of atoms in a crystalline solid are separated by distances on the order of angstroms; these layers can therefore act as a diffraction grating for radiation of similar wavelength (for instance, the Cu Kα and Mo Kα X-rays have wavelengths of 1.540562Å and 0.70930Å, respectively. Analysis of the resulting diffraction pattern produces an electron density map of atoms in the unit cell to allow for precise identification of atomic positions, bond lengths, angles, and size and symmetry of the unit cell. Diffraction is based on the interaction of a beam of X-rays with crystalline extended solids. For diffraction to occur, Bragg’s law must be satisfied. Diffraction occurs only at specific angles described by nλ=2dsinθ, where λ is the wavelength of energy from the X-ray source. This is of the same order of magnitude of the separation of lattice planes (referred to as d-spacing) [26]. Single crystal X-ray diffraction is performed on a single crystal mounted onto a cryoloop fiber using paratone oil or on a glass fiber with epoxy. Most experiments in this work involved air-sensitive materials which were mounted in paratone oil and frozen at 150K to minimize the potential for oxidation during data collection. In the diffraction experiment, the crystal is rotated in an X-ray beam at a wide range of angles to allow the various Miller planes to come into registry with Bragg’s law. From the position and intensity of the diffraction spots collected by the CCD detector, the unit cell symmetry and atom positions of the crystal can be determined. There are several programs that are utilized to determine the crystallographic structure. SAINT is used to integrate the data collected and SADABS is used to correct for absorption. Then, XPREP is used to determine the space group, set an approximate composition (based on EDS measurements carried out beforehand), and prepare the SHELX input files. The structure is solved with SHELXS and SHELXL. Information on space groups and symmetry can be found in references [27] and [28]. Powder X-ray diffraction, as the name implies, is used on a powdered sample. It is a destructive technique in which a small representative portion of a sample (including powders and single crystals) is ground together with a standard such as Si or LaB6. These standards are

10 crystalline phases with well-known diffraction patterns. The location of their diffraction peaks can be used to correct for misalignments in the diffractometer or errors in sample positioning. Si is commonly used because it has 3 diffraction peaks that typically don’t interfere with the powder pattern of the sample. If a standard peak does overlap with a sample peak, then another standard can be used. Powder diffraction can be used to determine the phases present in the sample, phase width of the system, unit cell parameters, and particle size. Phase analysis is used to determine whether or not the desired phase is present and if any impurity phases are present. The experimental powder pattern is compared to calculated patterns found in databases or calculated from single crystal data of an unknown phase. One of the downfalls of powder diffraction is the inability to detect the presence of phases if they comprise less than 5% of the total solid sample. This hinders the ability to detect magnetic impurities that may affect the magnetic data collected on a powder sample. Phase width is defined by the ability to substitute one element for another on one (or more) sites in the structure, without changing the structure. The expansion (or contraction, if a smaller element is substituted) of the unit cell should have a linear dependence on amount of substitution which is expressed as Vegard’s law. This can be explored using powder X-ray diffraction to determine how the unit cell parameters of the doped structures have changed from the original values.

2.5 Magnetic Measurements Magnetic behavior of a compound of interest is characterized using a Superconducting Quantum Interference Device (SQUID). There are three basic types of measurements that can be performed: temperature-dependent susceptibility, field-dependent magnetization measurements, and AC susceptibility measurements. Temperature dependent susceptibility measurements help to determine what type of magnetic behavior is present (ferromagnetic, anti-ferromagnetic, ferrimagnetic, and superconducting behavior) and any associated ordering temperatures; and the data reported as susceptibility (χ, emu/mol) vs. temperature (K). Zero-field cooled (ZFC) and field cooled (FC) measurements are performed to observe possible frustration in the system. ZFC measurements are obtained when the sample is cooled to low temperature (1.8K) and no applied field is present. The magnetic field is turned on and data is collected as the sample is

11 warmed up to room temperature. The FC measurement is obtained by cooling the sample with the applied field down to the temperature that was initially chosen. If frustration is present, AC susceptibility measurements are obtained. A low field (3-5 Oe) is applied along with different alternating frequencies (1-1000Hz); the temperature range where the frustration might be is scanned at each frequency. Magnetization measurements, χ vs. field (H, Orested), are used to understand the dependence of the magnetic behavior with changes in the applied field. Compounds containing unpaired electrons (such as rare earth compounds, some transition metal phases, metals and organic radicals) are paramagnetic. The magnetic moments of the unpaired electrons have a tendency to align with an applied magnetic field. The resulting magnetization (M) of a compound is dependent on the applied field (H); the ratio of these two values indicates the susceptibility (magnetic susceptibility χ) of the compound to align with the field (equation 2.5).

(2.5) The bulk susceptibility of compounds is composed of a Curie-Weiss contribution from localized unpaired electrons, Pauli-paramagnetism of conduction electrons (in metals), core diamagnetism, sample holder, and Van-Vleck paramagnetism. In most metallic compounds, Curie-Weiss and Pauli-paramagnetic behavior dominate. The other contributions --core diamagnetism, sample holder, and Van-Vleck paramagnetism-- are low enough and can be neglected for the majority of cases (equation 2.6).

(2.6) The temperature dependence of the magnetic susceptibility of a compound in its paramagnetic state (in which the electron magnetic moments are randomly oriented) is described by the Curie Law. When a field is applied, the random orientation of magnetic ions can align with the field. This magnetization is proportional to the applied magnetic field (see previous equation) and is also dependent on the total number of unpaired electrons and spin-orbit coupling if present. The magnetization M can be expressed as shown below (equation 2.7);

(2.7)

Substituting this value for M into the definition for susceptibility χ yields (equation 2.8)

(2.8) Where (equation 2.9)

(2.9) Then (equation 2.10)

(2.10) The Curie law assumes that there are no magnetic interactions between localized or itinerant unpaired electrons. However in many cases, including the materials studied here, coupling forces (exchange forces) do exist between unpaired spins. This is taken into account by the Curie-Weiss law. The total field HTOT is equal to the contributions from the applied field and the internal field created by magnetically coupled centers NW (equation 2.11) (2.11) Then M equals (equation 2.12)

(2.12) Solving for susceptibility (equation 2.13)

(2.13) In extended solids, interactions of many spin centers throughout the material may result in long range magnetic ordering. Ordering may be ferromagnetic with a transition at Tc or the Curie temperature. Before this transition, paramagnetic behavior dominates and all the spins are randomly oriented and cancel each other out. At Tc there is a rise in susceptibility and once the transition is complete, all of the spins are aligned parallel to one another and the susceptibility flattens out. When the inverse susceptibility is plotted, the high temperature data should be fit to equation 2.14 where 1/C is the slope of the line and θ is the Weiss constant. The Curie constant can be solved for by dividing by the slope. The moment per magnetic ion was calculated by taking the square root of the Curie constant and multiplying it by 2.84 (equation 2.14)

(2.14)

Antiferromagnetic materials are characterized by anti-parallel ordering of moments at TN or the Neel temperature. This behavior is commonly observed for rare earth metals (Ce, Pr, Nd) which exhibit similar TN temperatures, ranging from 10-16K [5]. Much like ferromagnetic materials, above the transition temperature, paramagnetic behavior dominates followed by a cusp indicating the ordering. The inverse susceptibility is plotted and the high temperature data is fit to equation 2.14. The Weiss constant should be negative and of similar magnitude to the Neel temperature. If the Weiss constant is very large (indicating strong magnetic coupling forces are present) but the magnetic ordering occurs at a much lower temperature, this is evidence of spin frustration. Canted antiferromagnetic materials and spin glass behavior are two special cases of antiferromagnetic behavior where their magnetic ions do not couple perfectly antiparallel to one another. Canted antiferromagnetic materials have spins that are aligned antiparallel, but along two different axes, instead of being aligned along the same axis and cancelling completely. The angle of canting can be determined using neutron diffraction [29]. Spin glass behavior is observed where there are competing ferromagnetic and antiferromagnetic interactions and can be caused by deformed lattices or a random distribution of ions that results in spin frustration. This is often indicated by splitting of the ZFC and FC measurements. When this splitting is observed, AC susceptibility measurements are performed. A low field and different frequencies are applied. Then, a temperature region is scanned at each frequency. The cusp of the antiferromagnetic transition is observed and should move if frustration is present with different applied frequencies. The instrument utilized in this research was a MPMS Quantum Design Magnometer. The sensitivity of this instrument to magnetic moments demands careful sample preparation to avoid the presence of impurities. Samples were prepared by selecting large single crystals (2-5 mg) that were previously screened with single crystal X-ray diffraction. Crystals were placed at the 3 cm mark of a 12 cm piece of Kapton tape which was folded in half. The sample was inserted into a plastic straw and placed into the magnometer. Temperature dependent susceptibility measurements were obtained on samples at low applied fields (100 Oe to 1T) and magnetization measurements above and below the observed transition.

2.6 Mössbauer Spectroscopy Mössbauer spectroscopy is a useful spectroscopic technique to study the interaction of certain nuclei. It is used most commonly for Fe, Eu, and Sn, but also for Ru, Sb, Te, I, W, Ir, Au, Gd, Dy, Er, Yb, Np with their surrounding environment, especially when there are multiple magnetic ions present [30]. The instrument requires the use of a Co γ radiation source and recoil free emission. Recoil free emission is achieved when γ radiation is sent towards the sample and the energy is low enough that there is no lattice vibration and the entire system recoils. When the sample is irradiated with γ radiation, the Mössbauer active nuclei undergo different energy level transitions as a γ ray is absorbed or emitted. This provides magnetic and electronic information about the local environment. These interactions split the energy levels within the system. There are three main types of interactions: isomer shift, quadrupolar interactions, and magnetic hyperfine splitting. These interactions stem from the hyperfine interactions between the Mossbauer active nuclei and the surrounding environment. These interactions are small compared to the nucleus but because of the high energy resolution, one can observe how these interactions change the nuclear energy levels [31]. The isomer shift is caused by the interactions of the s-electron density with the nucleus and the γ radiation. If there is a change in the electron density with the applied radiation, a shift of the peak is observed. This shift can be used to identify the oxidation states and the electronic properties of the system when compared to a standard, typically α-Fe. Other effects from the electron withdrawing groups and bonding properties of ligands can also be studied by the isomer shift of the compound [30]. Quadrupolar interactions arise from the non-spherical distribution of the electronic charge density around the Mössbauer active nuclei. This occurs when the nuclear angular momentum is greater than ½. When there is an asymmetric charge distribution, the electric field gradient changes and gives rise to splitting of the nuclear energy levels which results in two lines thus creating different alignments of the quadrupole moment. Magnetic splitting can be observed if there is a magnetic dipole interaction between the nuclear magnetic moment of the surrounding atoms. The ground state of 57Fe is composed of 3 ml = ± ½ and can be split into ∆ml = ± ½, /2 by the surrounding environment. A Mössbauer spectrum with a ratio of 3:2:1:1:2:3 intensities is observed.

Obtaining high-quality 57Fe Mössbauer measurements is somewhat difficult for samples that are very cerium-rich[32]. Enrichment of the sample is necessary because Ce is a γ-ray absorber and can reduce the interaction of γ-radiation with the 57Fe nuclei.

2.7 Differential Scanning Calorimetry (DSC) DSC is useful for examining phase transitions such as: melting, sublimation, and thermal decomposition. In this technique, the amount of heat needed to raise the temperature of the sample is compared to that needed to raise the temperature of a standard material. Differences occur when the sample undergoes exothermic or endothermic transitions. The change in heat capacity of the sample is compared to that of the standard. An associated thermogravimetric analyzer (TGA) can measure any weight loss or gain that occurs if the thermal event is associated with a change in composition. Powder diffraction can be used afterwards to observe any decomposition products [21]. There are many variables in these measurements which need to be controlled. First: the size and shape of the sample is important for the success of the experiment. Second the choice of atmosphere is important. Inert gasses are used so the sample does not oxidize during heating.

It is also possible to perform nitration and oxidation reactions in the TGA/DSC. N2 and O2 gas can be flowed over the sample during heating and cooling. Observation of an endothermic peak and a weight gain in the heating process indicating phase formation can help tailor large scale reactions to produce large quantities of the nitride or oxide [21]. Heating profiles are important; if the sample is heated and cooled too rapidly, lag time may occur between the changes occurring in the sample and the instrument recording these events [21].

2.8 Solid State Nuclear Magnetic Resonance (SS-NMR) Solution state nuclear magnetic resonance (NMR) is a useful technique to determine the structure of organic compounds, but in the solid state, it is a useful tool for understanding the structure and electronic properties of extended solids. There are four main interactions that can occur when a nucleus is placed in a magnetic field: Zeeman, direct dipole-dipole, chemical shift, and quadrupolar interactions[33]. A Knight shift is a term used to describe the effect of conduction electrons at the Fermi level on the position of the resonance and gives information about the electronic properties of the

16 system. Typically, materials that are metallic are shifted to higher, positive values and materials that are semiconducting are shifted to lower, negative values. Materials with shifts that are small relative to their metallic counterparts can be described as poor metals [35]. When nuclei have spins that are greater than ½, quadrupolar interactions may be present. This occurs when the quadrupolar moment is coupled to adjacent spin ½ nuclei. Broadening of the signal is caused by the Zeeman interaction and the quadrupolar interactions. Another component of this interaction is the dipole moment. This results in a 3-dimensional interaction. Applying a large external magnetic field results in a decrease of the Zeeman and dipolar interaction with the quadrupolar ineraction. However, even with an increase in the applied magnetic field, the dipolar interaction is not completely removed because averaging with spinning at the magic angle still does not remove this issue [34]. The experiment requires the use of a uniformly ground sample with a salt (NaCl, KCl, etc.) when there is a need to fill space or when the sample is conducting. Conducting samples can interfere with the spinning of the rotor because the conducing electrons can orient themselves with the external field. The sample and the salt are packed into either a 4mm or 2.5mm rotor made from Zirconium. The rotor is then inserted into a NMR, typically having a 500MHz (11.5T) magnet, and spun at a “magic angle.” Spinning at this magic angle removes the effects of anisotropy and first order quadrupolar interactions since the 3cos2θ-1 term in the Hamiltonian is equal to zero [34].

Chapter Three

Flux Growth of a New Cobalt-Zinc-Tin Ternary Phase Co7-xZn3-xSn8 and its Relationship to CoSn

3.1 Introduction Tin has been widely used as a flux; reactions in molten tin have led to the recent discovery of new phases such as RESn1+xGe1-x and OsB [36,37]. Tin melts have also enabled the growth of large crystals of known phases, including the Ba1-xKxFe2As2 superconductors, which have been of particular interest recently [38,39]. We have been investigating lowering the melting point of tin by combining it with other metals to form eutectic fluxes. The Zn/Sn phase diagram (Figure 3.1) is a simple eutectic system with no binary phases, featuring a eutectic at 15% Zn with a melting point of 199°C [6]. This eutectic is sold commercially as a lead-free solder (for example, it is marketed by Indium Corporation as Indalloy 201, recommended for soldering to high aluminum alloys). Interactions of metals with this Zn/Sn melt is therefore of interest to researchers in a number of fields, particularly given the worldwide interest in finding alternatives to lead-based solders [40,41]. In this study, cobalt was reacted in Zn/Sn eutectic to explore the growth of potentially magnetic materials. Complex peritectic behavior is seen during this reaction, similar to that observed for cobalt in pure tin flux (cobalt reacts in tin to from CoSn at high temperatures, which further reacts with the flux as the temperature is lowered, forming CoSn2 and CoSn3) [42]. In Zn/Sn flux, the binary CoSn forms at 1173K, and then reacts at lower temperatures to form a new ternary phase Co7+xZn3-xSn8 (-0.2

Figure 3.1 Zn/Sn binary phase diagram illustrating the lowered melting point of Zn from 42693K to 472K when combined with Sn in a 15%/85% atomic percent ratio [7].

3.2 Materials and Methods 3.2.1 Synthesis Co powder (Alfa-Aesar, 99.8%), Zn powder (Alfa-Aesar, 99.8%), and Sn granules (Alfa- Aesar, 99.9%) were combined in a 2:1.5:8.5 millimolar ratio in an alumina crucible. A second crucible containing silica wool was placed upside down above the reaction crucible to act as a filter during centrifugation. This setup was then placed in a fused silica tube, sealed under vacuum at 10-2 Torr, and then heated to 1173K in 6h, held at 1173K for 24h, cooled to 973K over 36h, held there for 24h, and then cooled to 593K over 36h. At 593K, the sample ws inverted and centrifuged to remove the excess molten Zn/Sn flux. The crystals removed from the alumina crucible had a square shaped appearance and were silver colored and reflective. In order to explore the phase width of this compound, reactions were carried out with different amounts of Co in the Zn/Sn eutectic. Co/Zn/Sn reactions were prepared with millimolar ratios y:1.5:8.5, which y varying in increments of 0.5 from 1.0mmoles up to 3.0mmoles. These reactions were carried out in alumina crucibles as described above. Products

19 of reactions with 1.0 or 3.0 millimoles of Co had a flatter, more plate-like appearance, noticeably different from the more blocky appearance of products of reactions with 2 millimoles of Co. After the structure of the ternary phase was determined, stoichiometric synthesis was attempted using 3.5 millimoles of Co, 1.5 millimoles of Zn, and 4 millimoles of Sn. These reactants were loaded into an alumina crucible, which was placed in a fused silica tube and sealed under vacuum at 10-2 Torr. The ampoule was then heated from 298 to 1173 K in 2h, held at 1173K for 6h, cooled to 300K in 6h and removed from the oven.

3.2.2 Elemental Analysis Elemental analysis was performed on all samples using a JEOL 5900 scanning electron microscope with energy-dispersive X-ray spectroscopy (EDXS) capabilities. Flux-grown crystals from each reaction were affixed to an aluminum SEM puck using carbon tape, and positioned so that flat faces were perpendicular to the electron beam. Samples were analyzed using 30kV accelerating voltage and accumulation time of 60s.

3.2.3 X-ray Diffraction For each single crystal experiment, a small single crystal was selected and mounted on a glass fiber using epoxy. Single crystal X-ray diffraction data were collected at room temperature using a Bruker AXS APEX2 CCD diffractometer with a Mo radiation source. Processing of the data was accomplished using the program SAINT. An absorption correction was applied to the data using the SADABS program [43]. Refinement of the structure was performed using the SHELXTL package [44]. X-ray diffraction data were collected for representative crystals from reactions with 1 and 2 millimolar amounts of Co reactant. The crystallographic data collection parameters are summarized in table 3.1, atomic positions and bond lengths are listed in tables 3.2-3.4

Table 3.1 Crystallographic parameters for the Co7+xZn3-xSn8 subcell and supercell structures.

Co7.2(1)Zn2.9(1)Sn8 Co6.8(4)Zn3Sn8 (subcell) (supercell Reactant ratio (Co/Zn/Sn) 2:1.5:8.5 1:1.5:8.5 Formula Weight (g/mol) 1556.96 1558.04 Crystal System Orthorhombic orthorhombic Space group Cmcm Pnma a(Å) 4.139(1) 12.5908(2) b(Å) 12.593(4) 11.6298(3) c(Å) 11.640(4) 8.2704(2) Z 2 2 Volume 606.7(3) 1211.02(5) Density, calc (g/cm3) 8.585 8.485 Temperature (K) 293

Radiation MoKα Index ranges -5≤h≤5 -20≤h≤20 -16≤k≤16 -18≤k≤15 -15≤l≤16 -11≤l≤13 Reflections collected 3318 12,538 Unique data/parameters 430/41 2618/99 (mm-1) 31.33 30.97 a R1/wR2 0.0301/0.0728 0.0374/0.0869

R1/wR2 (all data) 0.0314/0.0739 0.0580/0/0968 Residual peak/hole 1.37/-1.72 2.70/-2.75 (e- Å-3) a 2 2 2 2 2 1/2 R1Σ║Fo│-│Fc║/ΣFo; wR2=[Σ[w(Fo _Fc ) ]/S[Σ(Fo ) ]] .

Table 3.2 Atomic positions for the Cmcm subcell structure of Co7.2(1)Zn2.9(1)Sn8 subcell a Atom Wyckoff Occupancy X Y Z Ueq site Sn(1) 8g 0.505(2) 0.4373(2) 0.48324(8) ¼ 0.0076(3) Sn(2) 8f 0 0.34352(4) .02024(5) 0.0114(2) Sn(3) 4c 0 0.70920(6) 0.25 0.0112(2) Co(1) 8f 0 0.16053(8) 0.12905(8) 0.0110(3) Co(2) 8f 0.535(4) 0 0.5204(1) 0.1357(4) 0.0059(4) Zn(3)/Co(3) 4c 0.45(5)/0.55(5) 0 0.3430(1) 0.25 0.0082(5) Zn(1) 4a 0 0 0 0.0092(3)

Table 3.3 Atomic positions for the Pnma supercell structure of Co6.8(4)Zn3Sn8 a Atom Wyckoff Occupancy X Y Z Ueq site Co(1) 8d 0.08433(6) 0.13056(8) 0.37256(8) 0.0078(1) Co(2) 8d 0.22929(6) 0.13573(8) 0.1261(1) 0.0081(1) Co(3) 8d .904(4) 0.40499(7) 0.62746(9) 0.3789(1) 0.0078(2) Co(4) 4c 0.41104(8) 0.25 0.6253(1) 0.0066(2) Zn(1) 8d 0.24824(5) 0.00154(7) 0.3758(1) 0.0098(1) Zn(2) 8d 0.40301(8) 0.25 0.1251(1) 0.0114(2) Sn(1) 4c 0.08981(2) 0.52845(4) 0.12653(7) 0.0088(1) Sn(2) 4c 0.40468(3) 0.01162(4) 0.12422(7) 0.0086(1) Sn(3) 4c 0.88(5) 0.0376(3) 0.25 0.120(1) 0.0089(5) Sn(3a) 4c 0.12 0.044(1) 0.25 0.092 0.0089 Sn(4) 4c 0.04324(5) 0.25 0.63317(1) 0.0102(1) Sn(5) 4c 0.968(3) 0.26484(5) 0.25 0.4068(1) 0.0084(1) Sn(5a) 4c 0.032 0.260(1) 0.25 0.340(4) 0.0084 Sn(6) 4c 0.26867(5) 0.25 0.84411(7) 0.0097(1)

Table 3.4 Comparison between selected bond lengths in the subcell and supercell structures of Co7+xZn3-xSn8 Cmcm subcell Pnma supercell Bonds Length (Å) Equivalent bonds Length (Å) Zn(1)-Co(1) 2.519(1) x2 Zn(1)-Co(1), Zn(1)-Co(3) 2.552(1), 2.479(1) Zn(1)-Co(2) 2.616(1) x4 Zn(1)-Co(2) 2.600(2), 2.630(2) Zn(1)-Sn(2) 2.8758(7) x4 Zn(1)-Sn(2), Zn(1)-Sn(1) 2.820(1),2.868(1),2.890(1) Co(3)/Zn(3)-Sn(1) 2.528(1) x2 Zn(2)-Sn(5), Co(4)-Sn(3) 2.53(2), 2.547(2) Co(3)/Zn(3)-Co(2) 2.601(2) x2 Zn(2)-Co(2) 2.559(1) Co(3)/Zn(3)-Sn(3) 2.669(1) x2 Zn(2)-Sn(3), Zn(2)-Sn(4); 2.701(8), 2.771(2); Co(4)-Sn(3), Co(4)-Sn(4) 2.586(8), 2.600(2) Co(3)/Zn(3)-Sn(2) 2.674(1) x2 Zn(2)-Sn(2) 2.7724(5) Co(4)-Sn(1) 2.5766(5) Co(3)/Zn(3)-Co(1) 2.695(1) x2 Zn(2)-Co(1), Co(4)-Co(3) 2.672(1), 2.720(1)

Powder X-ray diffraction was used to explore the possible phase width associated with this compound. Samples of the solid product from each flux synthesis were ground with a small amount of silicon to act as an internal standard to allow accurate determination of unit cell parameters. Powder X-ray diffraction were collected using an original diffraction setup based on

a Huber imaging plate Guinier camera 670 that uses CuKα1 radiation (λ=1.54060 Å) with a Ge crystal monochromator. The accompanying JADE software was used to analyze the powder patterns. The theoretical powder pattern calculated from the crystal structure was compared to the data for productions of reactions at different millimolar ratios (Figure 3.2)

CoSn

3 mmol

2.5 mmol 2 mmol

1.5 mmol

1 mmol

Cmcm

Pnma

25 35 45 55 65 2-Theta

Figure 3.2 Powder X-ray diffraction patterns for solid products of Co/Zn/Sn flux reactions centrifuged at 593K, with Co:Zn:Sn mmol ratios of y:1.5:8.5 (values of y indicated on figure). Calculated powder patterns of Cmcm and Pnma structures of Co7+xZn3-xSn8 are plotted, as well as the pattern of CoSn, the commonly observed byproduct.

Refinement for each sample indicated very little change in unit cell parameters. Powder XRD data were also collected on samples of the product of attempted stoichiometric synthesis, and on flux grown Co7+xZn3-xSn8 samples after thermal analysis (Figure 3.3).

Pnma

Cmcm Thermal decomp product

Stoichiometric synthesis

CoSn2

CoSn

25 35 45 55 65 75 2-Theta

Figure 3.3 Powder X-ray diffraction pattern for the thermal degradation product of Co7+xZn3- xSn8 after heating to 973K under argon, and the PXRD pattern of the product of attempted stoichiometric synthesis of Co7+xZn3-xSn8. Calculated powder patterns of Cmcm and Pnma structures are plotted, as well as the pattern of byproducts Sn, CoSn, and CoSn2.

3.2.4 Magnetic Susceptibility Magnetic measurements were carried out with a Quantum Design MPMS SQUID magnetometer at temperatures between 2 and 300K. Crystals were ground into a fine powder and put into a capsule with the cap inverted to insure that the powder was tightly pressed and immobile. The capsule was placed in a plastic straw and placed into the magnetometer. Temperature dependent susceptibility data from 2 to 300K were collected at applied fields of 500-1000G, and field dependence data was collected at 3K. A superconducing transition was observed at 3.7K at low applied fields. This indicated a Sn impurity (likely from residual traces of flux) which was corrected for by increasing the applied filed to 1000G, above the critical field for Sn.

3.2.5 Thermal Analysis TGA/DSC analysis was performed using a TA Instruments Q600 to investigate the stability of the products and to study their formation in the flux. Data were collected on

Co7+xZn3-xSn8 samples synthesized from a 2:1.5:8.5 mmol Co/Zn/Sn reaction; individual crystals of the compound were selected and ground into powder. This powder was loaded into a small alumina cup placed in the instrument; argon gas was flowed at a rate of 100mL/min in order to prevent oxidation of the sample during heating. Each sample was heated at 10°/min to 1273K and cooled from 1273K to 298K at 5°/min. Powder X-ray diffraction data was taken afterwards to determine the thermal decomposition products. A DSC in-situ study of the flux reaction was carried out by preparing a scaled-down version of the 2:1.5:8.5 mmol mixture, with the total quantity of reactants reduced to a net mass of 150 mg to fit into the alumia sample cut. This miniature flux reaction was heated at 10°/min to 1273K and cooled from 1273K to 298K at 5°/min under a flow of argon gas.

3.3 Results and Discussion Our exploration of the reaction of cobalt in Zn/Sn eutectic flux was motivated by the fact that different phases were isolated from identical reaction compositions if they were centrifuged or quenched at different temperatures. This phenomenon is seen in the Co/Sn binary system; reaction of cobalt in an excess of tin yields CoSn, CoSn2, and CoSn3 which crystallize at different temperatures [45]. Similar behavior is also seen in more complex flux reactions. For instance, two structurally related layered phases Ce2PdGa12 and CePdGa6 can be crystallized from identical gallium flux reactions centrifuged at different temperatures [46]. Likewise, LaFe13-xAlx and La6Fe13-xAl1+x (comprised of similar building blocks) can be isolated at different temperatures from reactions of aluminum in La/Fe eutectic flux [46]. A 2:1.5:8.5 Co/Zn/Sn reaction yields CoSn if centrifuged above 873K; if it is instead allowed to cool to 593K and centrifuged, the new ternary phase Co7+xZn3-xSn8 is isolated. This behavior is apparent when a smaller scale flux reaction is monitored using DSC/TGA (Figure 3.4).

Figure 3.4 Differential scanning calorimetry data for the reaction of cobalt in Zn/Sn eutectic, taken as the flux reaction was cooled from 1273K to room temperature. The inset highlights the transition from CoSn to Co7+xZn3-xSn8.

At high temperature, cobalt reacts in the flux leading to crystallization of the binary phase CoSn, indicated by an exotherm at 1173K during cooling. This compound reacts with the Zn/Sn solution to form Co7+xZn3-xSn8 when cooled to 823K. Changing the amount of cobalt does not affect the temperature of the formation of the ternary phase. If the reaction is centrifuged at or near this temperature, the transition of one phase to another can be captured in progress; the resultant product features hexagonal rod-shaped crystals of the binary phase CoSn with protrusions of the ternary phase Co7+xZn3-xSn8 growing on them (Figure 3.5).

Figure 3.5 SEM image taken of the product isolated from the reaction of cobalt in Zn/Sn eutectic when centrifuged at 823K. Protrusions on the rods are the ternary phase (elemental analysis indicates presence of Co, Zn, and Sn; the hexagonal rods themselves only contain Co and Sn)

If the flux reaction is instead allowed to cool further, the ternary phase becomes predominant, with much smaller amounts of CoSn as a byproduct (Figure 3.4). An exotherm at 473K indicates freezing of the eutectic flux. Flux reactions cooled to 593K and centrifuged to remove the excess flux yield well- formed crystals of Co7+xZn3-xSn8, up to 2 mm on a side. The yield based on Co is approximately 80%, with small amounts of CoSn as a byproduct. The phase width of the title compound is very small (-0.2

CoSn and CoSn2 predominated (Figure 3.3).

Co7+xZn3-xSn8 can also be synthesized from a stoichiometric reaction of the elements, although not in pure form (contaminants include tin and CoSn; Figure 3.3). Powder XRD patterns were calculated based on the subcell and supercell crystal structures of Co7+xZn3-xSn8 (vide infra); these were compared to the observed powder pattern of the stoichiometric product. There are very subtle differences between the calculated powder patterns of the subcell structure (in space group Cmcm) and the supercell structure (one axis doubled, in space group Pnma). Additional peaks in the Pnma structure appear at low 2θ and are also found between 10° and 25° 2θ, 34° and 44° 2θ, and 46° and 56° 2θ. The powder pattern of the product of stoichiometric synthesis seems to indicate formation of the supercell, although the presence of diffraction peaks due to contaminants makes this unclear.

3.3.1 Co7+xZn3-xSn8 subcell and supercell crystal structures

Co7+xZn3-xSn8 forms in two related structures, depending on the initial Co concentration of the flux reaction. Single crystal data were taken for samples from several different Co/Zn/Sn reactions with ratios y:1.5:8.5 mmol. When 2 mmol Co were used, a subcell structure forms in the orthorhombic space group Cmcm (Figure 3.6a). On the other hand, products of reactions containing 1 or 3 mmol Co have a supercell structure in orthorhombic space group Pnma (Figure

3.6b). The axes of the unit cells are related as follows: asuper=bsub; bsuper = csub; csuper =2asub. While the amount of cobalt reactant in the flux does appear to reproducibly determine whether the structure forms in the subcell or supercell, it should be noted that the stoichiometry of the product does not change. Both the subcell and supercell structures have very similar elemental ratios (within estimated standard deviations; see Table 3.1).

Figure 3.6 Structure of Co7+xZn3-xSn8. Unit cells are indicated by dashed lines. Cobalt atoms are represented by blue spheres, zinc atoms by red spheres, and tin atoms by grey spheres; mixed Co/Zn sites are represented by purple spheres, and half-occupied cobalt sites indicated by blue hatched spheres a) Cmcm subcell structure, viewed down the a-axis. b) Pnma supercell structure, viewed down the c-axis.

To simplify consideration of the highly complex Co7+xZn3-xSn8 structure, Figure 3.6a and 3.6b depict the Cmcm and Pnma structures with the tin atoms removed, and highlight the doubled unit cell axis going from the subcell to the supercell.

Figure 3.7 Structure of Co7+xZn3-xSn8. Unit cells are indicated by dashed lines. Cobalt atoms are represented by blue spheres, zinc atoms by red spheres, and tin atoms by grey spheres; mixed Co/Zn sites are represented by purple spheres, and half-occupied cobalt sites indicated by blue hatched spheres. a) Cmcm subcell structure with tin atoms removed, viewed down the b-axis. b) Pnma supercell structure with tin atoms removed, viewed down the a-axis.

The change in symmetry and cell size appears to stem from the ordering of the cobalt and zinc on individual 4c sites in the supercell (atoms Zn(2) and Co(4) in table 3.3). These sites

30 correspond to a single 4c site in the subcell, which refines as occupied by 45(5)%/55% (atom Zn(3)/Co(3) in Table 3.2 and Figure 3.6a). Additional differences are found in other cobalt sites, which form a zig-zag chain running parallel to one axis in the structure (along the a-axis in the subcell; along the c-axis in the supercell). In the subcell, this chain is comprised of Co(1) and Co(2) atoms (both in 8f sites). The Co(2) site is half-occupied. It is adjacent to a tin site (Sn(1)), which has a symmetry equivalent 0.56 Å away which mandates half-occupancy for this tin atom. One of these symmetry equivalents is a very short distance (2.2 Å) from the Co(2) site, which requires half occupancy for the cobalt atom. In the supercell, a similar situation occurs with tin atoms occupying split sites leading to short Co-Sn distances. However, this causes ordered vacancies in the cobalt zig-zag chain in the supercell, instead of the random partial occupancy seen in the subcell.

3.3.2 Structural relationship to CoSn

The Co7+xZn3-xSn8 structure features predominately heteroatomic bonding; there are no Zn-Zn or Sn-Sn bonds, and while there are Co-Co bonds, cobalt exhibits far shorter bonds to neighboring Zn and Sn (see Table 3.4). It is difficult to break the overall structure down into coordination polyhedral as is often done for more polar intermetallics containing highly electropositive metals such as alkaline earths or lanthanide elements, which often exhibit highly symmetric bonding environments that can be viewed as distinct building blocks. While such easily notable coordination spheres are not present in the title compound, some structural moieties are found which are similar to those in the CoSn binary phase. In particular, the cluster of Sn and Co atoms surrounding the Co/Zn mixed site (site 4c in the subcell) is also found in CoSn; this is shown in figure 3.8. The Co-Sn and Co-Co bond lengths in this moiety and throughout the structure are similar to those in CoSn, which features Co-Co bonds of 2.639 Å and Co-Sn bonds of 2.619 and 2.639 Å [48]. Comparable bond lengths are also found in ternary phases such as Mg2Co3Sn10; Co-Sn bonds in this compound range from 2.604 to 2.772 Å [49].

The presence of these similar structural building blocks in Co7+xZn3-xSn8 and CoSn may enable CoSn to act as a template for the formation of the ternary phase. The conversion of the binary phase to the ternary phase may be attributed to the presence of Zn in the solution. As the binary phase begins to precipitate out of the flux, the remaining solvent mixture becomes richer in zinc. As the reaction temperature is lowered the zinc begins to migrate into the CoSn

31 structure, substituting for Co and leading to formation of the ternary phase from the feedstock of CoSn. Reactions that were carried out below the formation temperature of CoSn produced this phase as well; however, the yield was much lower and CoSn3 was found as a competing phase.

Figure 3.8 Comparison between the structures of CoSn and Co7+xZn3-xSn8. a) The hexagonal CoSn structure viewed down the c-axis; cobalt atoms indicated by blue spheres. b) The Cmcm structure of Co7+xZn3-xSn8, viewed down the a-axis. c) The common building block of the cobalt site (or Co/Zn site in the ternary phase) and its coordination environment.

3.3.3 Magnetic Characterization Cationic cobalt in coordination complexes and ionic compounds exhibits a magnetic moment associated with its oxidation state and the local ligand field environment. The magnetism of cobalt in intermetallics is more complex due to the formation of bands by the interacting d-orbitals. The magnetic moment associated with cobalt will depend on the filling of these bands and their vicinity to the Fermi level. In general, cobalt atoms in cobalt-poor intermetallics do not possess magnetic moments. Phases such as Mg2Co3Sn10+x and La3Co4Sn13 exhibit only Pauli paramagnetism due to conduction electrons [49,50]. Cobalt-rich intermetallics do feature cobalt moments; the hard magnetic properties of SmCo5 are due to the ordering of magnetic moments on Co 91.4-1.5 µB per Co) [51]. Magnetic moments ranging from -.3 to

1.9µB per Co are observed for phases such as Co3Sn2S2, R6Co13Sn, and RCo12B6 (R = rare earth) [51, 52].

The behavior of Co7+xZn3-xSn8 is of interest since it forms in two slightly different structure types and its stoichiometry is in the intermediate regime between “cobalt-rich” and “cobalt-poor” intermetallics. Magnetic susceptibility data were collected on samples crystallizing in Cmcm or Pnma structures; the data are shown in figure 3.9.

Figure 3.9 Magnetic susceptibility data for the Co7+xZn3-xSn8 phase in both subcell and supercell structures.

Both datasets appear to be dominated by Pauli paramagnetism, with Curie tails at low temperatures possibly due to traces of cobalt oxides formed on particle surfaces. A modified Curie-Weiss law was used to fit the data (χ = (C/T-θ) + χo, where C and θ are the Curie and Weiss constants corresponding to the cobalt moments, and χo is the temperature independent Pauli paramagnetism). The cobalt moments derived from the Curie constants were negligible for both samples (0.003 and 0.001µB per cobalt atom for the subcell and supercell structured samples respectively), as were the Weiss constants (below 3.0K in both cases). The Pauli paramagnetism values of 0.00018(4) emu/mol for the subcell structured sample and 0.00010(8) emu/mol for the supercell structured sample are in the 10-3 to 10-4 emu/mol range expected for intermetallics.

3.4 Conclusion A new ternary phase was found in the Co/Zn/Sn system, growing as well-formed crystals from a Zn/Sn eutectic flux. Quenching reactions at different temperatures played an integral role

33 in discovery of this compound, since it forms as a low temperature phase from the reaction of CoSn with the Zn/Sn melt. This emphasizes the importance of exploring the effects of temperature in metal flux reactions. The reason is not clear as to why the cobalt reactant concentration in the flux determines the formation of Co7+xZn3-xSn8 in the subcell or supercell structure. Our initial reactions of other metals (iron and nickel) in Zn/Sn flux indicate they form similar compounds, both crystallizing in the Cmcm subcell structure. Compared to the cobalt analog, the Fe7+xZn3-xSn8 phase has a slightly larger unit cell parameters (a = 4.1171(5) Å, b=12.605(3) Å, c=11.621(3) Å) and Ni7+xZn3-xSn8 exhibits slightly smaller cell parameters (a=4.117195) Å, b=12.482(1) Å, c=11.616(1) Å), as expected from their relative atomic radii. The formation of these intermetallics in Zn/Sn eutectic flux reactions may have important implications on the use of Zn/Sn solder with transition metal alloys.

Chapter Four

Magnetic behavior of Ce21Fe8M8C12 (M = Si, Ge, Sn, Pb)

4.1 Introduction Solvent based growth of intermetallics has become a popular method for growth of new intermetallics. Examples of this include RE(AuAl2)nAl2(AuxSi1-x)2 [53],CaMgSi [54], and

SmNiAl4Ge2 [55] using either a single metal as a solvent or a eutectic. La-Ni has provided a rich synthetic vein for the growth of new structures such as La2Ni2-xRuxAl[56], La14Sn(MnC6)3[57], and La6Fe13-xAl1+x [58]. Of those products, La21Fe8Sn7C12 proved to be particularly interesting, as it was an excellent example of a compound exhibiting geometric frustration. The presence of ideal tetrahedra of iron atoms with antiferromagnetic coupling forces between them led to spin glass behavior. By synthesizing analogs of this structure type, further exploration into the magnetic properties can be achieved and the influence of additional paramagnetic contributions from the rare earth can be studied.

The La21Fe8Sn7C12 structure type is fairly flexible. It is able to incorporate a variety of substitutions in the La site (Ce and Pr analogs can be made); iron can be substituted for Mn, and a large number of heavy main group elements can be incorporated on the Sn site; see Table 4.1. Changing the rare earth element from La to Ce resulted in the formation of the same structure, but some of the main group reactants (Bi, Te, Sb) were not reactive in the Ce-Co eutectic flux. Also, since Ce is a smaller R element, a Si analog was able to be formed that contained Al on its own crystallographic site. The focus of this work was on characterizing the magnetic properties of Ce21T8Sn7C12 (T = Fe, Mn) and comparing the properties to those of the La analogs. The Mn structure exhibited paramagnetic behavior, whereas the Fe containing structure exhibited a ferromagnetic-like transition at 140K, compared to the La analogs that exhibited spin glass behavior.

Table 4.1 Unit cell parameters for R21T8M7C12 analogs. Analogue Unit Cell Å

La21Fe8Te4.2Al2.8C12 16.2911(7)

La21Fe8Ge4.9Al2.1C12 16.1589(4)

La21Fe8Sn7C12 16.6032(4)

La21Fe8Sb7C12 16.4101(2)

La21Fe8Bi7C12 16.2124(5)

La21Mn8Sn7C12 16.7435(2)

La21Mn8Bi7C12 16.6072(5)

La21Mn8Sb7C12 16.4829(4)

La21Mn8Ge6.2Al0.8C12 16.2259(3)

La20.02(1)Mn8Te7C12 16.4455(7)

Ce21Fe8Si6AlC12 15.717(3)

Ce21Fe8Ge6.8Al0.2C12 15.770(2)

Ce21Fe8Sn7C12 16.211(1)

Ce21Fe8Pb7C12 16.239(1)

Ce21Fe8Bi7C12 16.282(1)

4.2 Materials and Methods 4.2.1 Synthesis

Ce21Fe8M7C12 (M = Si, Ge, Sn, Pb) were synthesized using starting materials that were stored and handled in an argon-filled dry-box. Powders of these elements; C (95-97%, Strem Chemcials), Si(99%, Strem Chemicals), Fe (99%, Alfa Aesar) , Sn (99%, Alfa Aesar), Ge (Alfa Aesar, 99%), and Pb (Alfa Aesar 99%) were used in synthesis. Ce-Co eutectic was made from Ce ingot (Alfa Aesar, 99%) and Co pieces (Alfa Inorganics 99%). These were combined in a

76%:24% atm ratio and arc melted on a water-cooled copper hearth into a button that was turned over and re-melted several times to ensure homogeneity of the eutectic flux material. Starting materials of Fe or Mn, C, M (M = C, Si, Sn, Ge, Pb) were combined in a 1:1:0.7 millimolar ratio and sandwiched between layers of Ce-Co eutectic (for a total of 1.5g) with more eutectic flux material on the top layer compared to the bottom layer. The reactants were placed into an alumina crucible along with a second alumina crucible that was filled with Fiberfrax and inverted above the reaction crucible to act as a filter during centrifugation. The alumina crucibles were placed into a silica tube, which was fused under a vacuum of 10-2 Torr. The ampoule was then heated to 950°C in 3 hours, held at this temperature for 12 hours, and then cooled to 850°C in 10 hours. The reaction mixtures were subsequently annealed for 48 hours at 850°C and then cooled to 600°C in 84 hours. At 600°C the ampoule was removed from the furnace, quickly inverted, and placed into a centrifuge to decant the molten flux. Products from these reactions were stored in an argon-filled dry-box until characterization in order to prevent any surface oxidation.

4.2.2 Elemental Analysis Elemental analysis was performed on all samples using a JEOL 5900 scanning electron microscope with energy-dispersive X-ray spectroscopy (EDS) capabilities. Flux-grown crystals from each reaction were affixed to an aluminum SEM puck using carbon tape, and positioned so that flat faces were perpendicular to the electron beam. Samples were analyzed using a 30kV accelerating voltage and an accumulation time of 60s. The carbon content was not able to be determined due to the limitation of EDS with light elements. The samples were also checked for contamination by Al, which may have been leached from the crucible. To test for incorporation of Al, all site occupancies were allowed to vary in the single-crystal XRD final refinement cycles.

4.2.3 X-ray Diffraction Selected single crystals were mounted on cryo loops using paratone oil. The X-ray intensity data were collected at 150K on a Bruker SMART APEX2 CCD diffractometer equipped with a Mo-target X-ray tube (λ= 0.71073Å). The data sets were recorded as ω scans at 0.3° stepwidth and integrated with the Bruker SAINT software. Data were corrected for

absorption effects using the multiscan method (SADABS)[43]. The structures were solved in the centrosymmetric space group Fm-3m (no225) and refined by full matrix least squares procedures on |F2| using the SHELX-97 software package [44]. In Table 4.2 the collection data are listed, and Table 4.2 contains the atomic positions for each of the analogs.

Table 4.2: Collection parameters for all the analogues of Ce21T8M7C12

Ce21Fe8AlSi6C12 Ce21Fe8Ge6.8Al0.2C12 Ce21Fe8Sn7C12 Ce21Fe8Pb7C12 Ce21Mn8Sn7C12 Formula Weight 3728.9 4031.8 4362.2 4982.7 4356.9 Crystal System cubic cubic cubic cubic cubic Space Group Fm-3m Fm-3m Fm-3m Fm-3m Fm-3m a, Å 15.718(4) 15.770(2) 16.211(1) 16.239(1) 16.264(1) Volume 3883.07 3922.0 4260.8 4282.5 4302.1 Temp K 150

Radiation MoKα Index Ranges -17≤h≤17 -20≤h≤19 -14≤h≤21 -21≤h≤21 -21≤h≤20 -17≤k≤17 -20≤k≤9 -16≤k≤20 -21≤k≤21 -21≤k≤21 -17≤l≤17 -20≤l≤20 -21≤l≤19 -21≤l≤21 -21≤l≤21 Reflections 8100 5823 6358 12120 12219 Collected Unique 182/21 290/21 313/21 310/21 321/21 Data/parameters µ (mm-1) 27.12 31.749 28.53 51.789 27.91 R1/wR2 0.0158/0.0312 0.0163/0.0408 0.0183/0.0402 0.0205/0.0523 0.0123/0.0253 R1/wR2 (all 0.0181/0.0318 0.0167/0.0408 0.0186/0.0404 0.0206/0.0523 0.0127/0.0255 data) Residual 0.877/-0.772 1.255/-2.814 1.237/-1.338 2.803/-1.671 0.968/-0.609 Peak/hole (e-Å-3)

Table 4.3 Atomic positions for analogs of La21Fe8M7C12

Ce21Fe8Si6AlC12 Atom Wyckoff site x y z ueq Ce1 48h 0 0.16534(3) 0.16534(3) 0.0089(2) C1 48g 0.1018(7) ¼ ¼ 0.010(2) Fe1 32f 0.19267(7) 0.19267(7) 0.19267(7) 0.0088(5) Ce2 32f 0.36799(3) 0.36799(3) 0.36799(3) 0.0040(1) Si1 24e 0.2894(3) 0 0 0.0109(1) Ce3 4b ½ ½ ½ 0.0206(6) Al1 4a 0 0 0 0.012(2)

Ce21Fe8Sn7C12 Atom Wyckoff site x y z ueq Ce1 48h 0 0.1887(2) 0.1887(2) 0.0055(1) C1 48g 0.1059(7) ¼ ¼ 0.006(1) Fe1 32f 0.19408(6) 0.19408(6) 0.19408(6) 0.0043(3) Ce2 32f 0.36438(2) 0.36438(2) 0.36438(2) 0.0047(1) Sn1 24e 0.28997(6) 0 0 0.0065(2) Ce3 4b ½ ½ ½ 0.0083(3) Sn2 4a 0 0 0 0.0094(4)

Ce21Fe8Ge6.8Al0.2C12 Atom Wyckoff site x y z ueq Ce1 48h ½ 0.16519(2) 0.16519(2) 0.0045(1) C1 48g 0.1020(5) ¼ ¼ 0.006(1) Fe1 32f 0.19278(6) 0.19278(6) 0.19278(6) 0.0037(3) Ce2 32f 0.36727(2) 0.36727(2) 0.36727(2) 0.0039(1) Ge1 24e 0.29071(8) 0 0 0.0052(2) Ce3 4b ½ ½ ½ 0.0121(4) Ge1 4a 0 0 0 0.008(1) Al1 4a 0 0 0 0.008(1)

Table 4.3 Continued Atomic positions for analogs of La21Fe8M7C12

Ce21Fe8Pb7C12 Atom Wyckoff site x y z ueq Ce1 48h 0 0.16888(3) 0.16888(3) 0.0025(2) C1 48g 0.1054(8) 0.25 0.25 0.004(2) Fe1 32f 0.19401(9) 0.19401(9) 0.19401(9) 0.0014(4) Ce2 32f 0.36373(3) 0.36373(3) 0.36373(3) 0.0020(2) Pb1 24e 0.29107(5) 0 0 0.0033(3) Ce3 4b ½ 0 0 0.0056(5) Pb2 4a 0 0 0 0.0098(6)

Ce21Mn8Sn7C12 Atom Wyckoff site x y z ueq Ce1 48h 0 0.16911(1) 0.16911(1) 0.0068(1) C1 48g 0.1046(3) ¼ ¼ 0.007(1) Mn1 32f 0.19469(4) 0.19469(4) 0.19469(4) 0.0049(2) Ce2 32f 0.36537(1) 0.36537(1) 0.36537(1) 0.0055(1) Sn1 24e 0.28904(4) 0 0 0.0065(1) Ce3 4b ½ ½ ½ 0.0100(2) Sn2 4a 0 0 0 0.0124(3)

4.2.4Magnetic Susceptibility Magnetic measurements were carried out with a Quantum Design MPMS SQUID magnetometer at temperatures between 2 and 300K. Single crystals were used for the measurements. The crystal was placed at the 3 cm mark of a 12 cm piece of kapton tape. This tape was folded in half to position the sample in the center of the resultant 6cm length piece. The tape containing the crystal was placed in a plastic straw and then placed into the magnetometer. Both zero-field cooled (ZFC) and field-cooled (FC) measurements were collected at an applied field of 100Oe with a temperature range of 2-300K, and field dependence data were collected at different temperature depending on the transition temperature of the specific analogue.

4.3 Results and Discussion 4.3.1 Synthesis Synthesis of this phase was accomplished using Ce-Co eutectic flux. A few changes were made from the synthesis of La21Fe8M7C12. Adding additional Ce to the reaction did not increase the yield; rather, it made it more difficult for the phase to be formed. To combat the formation of Ce(Fe1-xCox)2 as the main product, additional Fe was added (1mmol vs. 0.8mmol) to promote the formation of Ce21Fe8M7C12 phases. Crystals of these phases crystallize in well faceted spheroidal shapes. Since the pseuodbinary phase is visually distinct (triangular cubic shape) it was easy to separate the desired phase without the use of elemental analysis, but elemental analysis was necessary to confirm the identity of the phase. In Figure 4.1, a large single crystal of Ce21Fe8Ge6.8Al0.2C12 is shown. This crystal exhibits truncated cuboidal growth in the optical microscopy image. Crystals of this phase typically grow in sizes of approximately 1mm in diameter.

Figure 4.1 Optical microscopy image of Ce21Fe8Ge6.8Al0.2C12. It exhibits truncated cuboidal growth. This size of this crystal is approximately 3mm.

The reactivity of the Fe and Mn in Ce-Co eutectic is very similar to that in La-Ni eutectic; there is almost 100% incorporation of Fe into the structure. Trace amounts of Co were observed in the EDS, but that was attributed to the overlap of the Fe Kβ with the CoKα peaks. In the Mn analogs, EDS analysis suggests possible doping of the structure with Co on the Mn sites, but not in significant quantities.

Synthesis of Ce21Fe8AlSi6C12 was accomplished using Si powder. Aluminum was initially leached from the alumina crucible. When Al was directly added to the reaction the yield went up significantly. One cannot eliminate the possibility of Al inclusion on the other Si site as well. All of these products crystallize in very large crystals between 1mm3 and 3mm3.

4.3.2 Crystal Structure This structure contains 4 Fe atoms in a tetrahedral arrangement capped by C atoms. The M1 site is coordinated by 9 R atoms in a capped square anti-prism coordination and the M2 site is weakly coordinated by 12 R atoms in a cubeoctahedral arrangement (figure 4.2). The bond lengths associated with these analogs compare well with literature values. For example, Ce-Ce bond distances of 3.720(1) Å -3.8950(5) Å for Ce21Fe8Sn7C12, compare well with literature lengths of 3.477 Å -3.852 Å [57] from Ce2C3. Ce-Sn bond lengths in this compound range from 3.3351(6) Å -3.872Å. Bond lengths that are observed in literature for Ce-Sn range from 3.313 Å

-3.528 Å (Ce2Sn5[58])). The longer bond length of 3.872 Å is associated with the Sn2 site which is weakly coordinated to the surrounding R. All bond lengths associated with R21T8Sn7C12 (R = La, Ce; T = Mn, Fe) are in table 4.4 and for all other Ce analogs, bond lengths are found in table 4.5.

Figure 4.2 Unit cell representation of RE21T8M7C12 where green spheres represent the RE atom, blue spheres the main group element (M), red spheres represent Fe or Mn (T), and black spheres represent C atoms.

In Ce21Fe8AlSi6C12, the 4a site is occupied by another main group element Al, rather than Si. This is an unexpected result. When the occupancies of the M site were varied, 100% occupancy was observed for Al, but the inclusion of Si cannot be ruled out. Replacing the M1

site with Si, resulted in a lower R1 value, but the site was only 98% occupied suggesting that the site may be mixed with Si and Al. Elemental analysis suggests a composition of approximately Al 3.6%, Si 19.5%, Fe 14.0%, and Ce 62.9%. Bond lengths between Ce-Si and Ce-Al are similar, 3.095Å -3.154 Å and 3.100 Å -3.301 Å respectively[59] [60]. The bond lengths reported for this structure range from 3.183(2) Å -3.310(4) Å, which suggests that mixing occurs on the M1 site. Bond lengths of this compound as well as the other analogues are shown in the table below (table 4.4 and table 4.5)

Table 4.4 Bond lengths of La21T8Sn7C12 vs. Ce21T8Sn7C12 where T = Mn, Fe

Distance Å La21Fe8Sn7C12 Ce21Fe8Sn7C12 La21Mn8Sn7C12 Ce21Mn8Sn7C12 R-R 3.7620(15)- 3.720(1)- 3.7484(9)- 3.7925(5)- 3.9897(7) 3.8950(5) 4.055(5) 3.9112(3) R1-M1 3.4444(8) 3.3689(7) 3.4764(5) 3.3719(4) R2-M1 3.4122(7) 3.3351(6) 3.4302(4) 3.3362(4) R3-M1 3.5042(13) 3.405(1) 3.5356(8) 3.4309(6) R1-M2 3.9897(1) 3.872 4.0455(5) 3.890 R1-C 2.618(10) 2.531(6) 2.602(9) 2.521(4) R2-C 2.733(2) 2.666(1) 2.776(2) 2.698(1) R1-T 3.3033(17) 3.19991) 3.360(1) 3.2207(8) R2-T 3.1520(9) 3.0691(7) 3.1911(5) 3.1007(5) T-T 2.554(4) 2.564(2) 2.502(3) 2.544(1) T-C 1.915(11) 1.919(6) 1.949(7) 1.940(4)

Table 4.5 Bond lengths of analogs of La21Fe8M7C12

Distance Å Ce21Fe8AlSi6C12 Ce21Fe8Ge6.8Al0.2C12 Ce21Fe8Pb7C12 Ce-Ce 3.675(1)-3.8374(9) 3.6842(7) -3.8477(6) 3.726(1)-3.8974(6) Ce1-M1 3.249(3) 3.2718(9) 3.3850(6) Ce2-M1 3.183(2) 3.1954(1) 3.3447(7) Ce3-M1 3.310(4) 3.300(5) 3.3929(8) Ce1-M2 3.675 3.684 3.878 Ce1-C 2.469(8) 2.482(5) 2.530(9) Ce2-C 2.665(2) 2.662(1) 2.659(2) Ce1-T 3.088(1) 3.102(1) 3.203(1) Ce2-T 3.068(1) 3.0629(7) 3.05891) T-T 2.549(3) 2.551(2) 2.571(4) T-C 1.915(9) 1.918(6) 1.92(1)

As expected, the bond lengths associated with the Ce analogs appear to behave as expected for the lanthanide contraction. For example, the bond lengths between La atoms in

La21Fe8Sn7C12 are 3.7620(15) Å - 3.9897(7) Å, which are longer than the Ce bond lengths of 3.720(1) Å-3.8950(5) Å . Replacing the Fe with Mn increases those bonds to 3.7925(5) Å - 3.9112(3) Å as expected to accommodate the slightly larger transition metal. Bond lengths associated with Ce increased as the size of the main group element increased.

4.3.3 Magnetic Properties In lanthanum-containing intermetallics, the La3+ cations do not have any unpaired f- electrons, so these ions do not contribute to the magnetic behavior of the compounds. Previous

work on the La21Fe8Sn7C12 system allowed for the focus to be on the magnetic behavior of the iron atoms. The frustrated Fe4 tetrahedra contained in this compound exhibited spin glass behavior as observed in temperature dependent susceptibility and AC measurements [61]. If Fe is replaced with Mn, the lower magnetic moment and weaker coupling forces between Mn atoms

44 in the Mn4 tetrahedra are not sufficient to induce magnetic frustration, so paramagnetic behavior is observed in the La21Mn8M7C12 compounds [60].

Cerium analogs Ce21T8M7C12 were synthesized to explore magnetic behavior and the influence of both the transition metal and a paramagnetic rare earth ion. Temperature dependent susceptibility were collected on all samples. Each of the temperature dependent susceptibility measurements for the iron-containing analogs consistently indicated the presence of what appears to be a ferromagnetic ordering at relatively high temperatures (120K to 140K) followed by a Curie tail at low temperatures caused by Ce3+ impurities after the transition [62]. The high temperature of the magnetic ordering indicates that it is likely due to the iron atoms. The ferromagnetic nature of the ordering is somewhat surprising, given that the iron atoms in the lanthanum analogs were antiferromagnetically coupled. However, the presence of the cerium moments may contribute to a ferromagnetic coupling mechanism in the Ce21Fe8M7C12 phases. The data above the ordering temperature could not be fit to the Curie-Weiss law (Figure 4.3)

0.005

0.0045 Ce21Fe8Pb7C12

0.004 Ce21Fe8Ge7C12

0.0035

0.003

0.0025

0.002 (emu/mol Ce) (emu/mol χ χ χ χ 0.0015

0.001

0.0005

0 0 50 100 150 200 250 300 350 400 Temperature (K)

Figure 4.3 Temperature dependent susceptibility measurements on crystals of Ce21Fe8Pb7C12 and Ce21Fe8Ge7C12 showed a ferromagnetic like transition at approximately 140K and 130K respectively.

Data collected on samples of Ce21Fe8Si7-xAlxC12 prepared in different ways reveals possible influences of aluminum incorporation on the magnetic behavior of the material.

Ce21Fe8Si6AlC12 synthesized in the absence of deliberately added aluminum (Al leached from the crucible) exhibits a ferromagnetic-like transition at 120K, similar to the other Ce21Fe8M7C12 analogs. When this phase was synthesized with aluminum intentionally added to the reaction, it was expected that the magnetic behavior would be the same. Instead, the product showed paramagnetic behavior with a low temperature Curie tail. This is depicted in Figure 4.4. The ferromagnetic-like behavior is suppressed when Al is directly added to the reaction. This suggests possible doping of Al onto Fe sites, reducing the amount of Fe-Fe bonding and reducing the ability of these atoms to couple magnetically. Similar behavior was noted in Ce33Fe14- xAlxB25C34

Figure 4.4 Temperature dependent susceptibility data for Ce21Fe8Si6AlC12, where the red squares represent the sample with just silicon added and black squres where Si and Al were directly added to the reaction.

Both Mn and Fe analogs of Ce21M8Sn7C12 (M = Mn, Fe) were synthesized so their magnetic behavior could be compared to that of the corresponding lanthanum analogs.

Ce21Fe8Sn7C12 exhibits ferromagnetic-like ordering (TC = 130 K) and a Curie tail at low temperature. The Mn analog does not show magnetic ordering; instead, it exhibits paramagnetic behavior. This transition from strong coupling in the Fe analog to weak coupling in the Mn compound is similar to what is seen in the lanthanum containing phases. La21Fe8Sn7C12 has strong coupling between its Fe atoms (it is antiferromagnetic and spin-frustrated, but the high

Weiss constant indicates a strong coupling force); La21Mn8Sn7C12, on the other hand, exhibits low magnetic moments and weak coupling forces between Mn atoms, with a corresponding lack of magnetic ordering (figure 4.5). Because most of the high temperature susceptibility data for the Ce21Fe8M7C12 phases cannot be fit to the Curie-Weiss law, the contributions of the Ce and Fe atoms to the magnetic behavior cannot be explicitly determined. It can be inferred based on comparison with the La analogs that the cerium ions cause a change in the magnetic behavior such that a relatively high temperature ferromagnetic transition is observed. Measurements on

Ce21Mn8Sn7C12 further support that this transition is due to the magnetic moments on iron atoms, but the possible additive interaction between Ce and Fe cannot be ignored (figure 4.5) [46]. Ce magnetic behavior was corrected by using the paramagnetic value reported for the La analogs.

Figure 4.5 Temperature dependent magnetic susceptibility measurements of La21Fe8Sn7C12 compared to the Ce analog. They have distantly different behavior.

Figure 4.6 Temperature dependent susceptibility measurements of La21Mn8Sn7C12 compared to the Ce analog. Both compounds exhibited similar magnetic behavior where the Mn does not participate in the magnetic behavior.

4.4 Conclusion

The R21T8M7C12 (R = Ce, La; T = Fe, Mn; M = Si, Ge, Sn, Pb, Bi, Sb, and Te) is a very versatile structure type that allows different R, T, and M elements to be incorporated. Products with this structure type have already been observed from reactions in other eutectics (Pr/Co and

Nd/Co). Comparison of these analogs against their La counterparts; particularly R21T8Sn7C12 (R = La, Ce; T = Fe, Mn) is useful in understanding magnetic behavior. Since this structure does not contain boron, and large crystals of this phase can be grown, neutron diffraction studies would be ideal. Due to the limitations of time and resources, neutron diffraction time was applied for, but not obtained. Studies of the Fe clusters could be made using Mossbauer spectroscopy, but sample preparation cost ($4.24/mg) is a limitation to this.

Chapter Five

A Tale of Two Metals: New Cerium Iron Borocarbide Intermtallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes

5.1 Introduction Metal flux synthesis has proven to be a vital tool for the discovery and crystal growth of new solids. This synthesis technique allows for the solvation of refractory elements in low melting metal solvents; the low reaction temperatures afforded by this method can promote the growth of new intermetallic compounds with complex structures [2] Expanding from single metal solvents to mixed fluxes comprised of two metals can allow for lowered solvent melting points through formation of eutectics; it also increases the number of reactants that are soluble and reactive in the flux. The use of mixed metal eutectic fluxes has aided the discovery of a broad range of new intermetallic phases and Zintl phase compounds such as Ca21Ni2Zn36 (grown in Ca-Zn eutectic), Co7Zn3Sn8 (grown in Zn-Sn eutectic), and LiCa2C3H (grown in Ca-Li mixture) [63, 64, 65]. Our explorations of reactions in La-Ni eutectic flux (mp 517°C) have yielded several new multinary intermetallics comprised of lanthanum, a transition metal, and one or more main group elements; examples include La21T8M7C12 (T = Mn, Fe; M = Ge, Sn, Sb, Te,

Bi), La2Ni2-xRuxAl, and La14Sn(MnC6)3 [61, 66, 67]. Expanding on the La-Ni eutectic work, Ce-T and Pr-T (T = Fe, Co, Ni) eutectic fluxes are now being investigated as synthesis media for magnetic intermetallic materials. Phases containing both a paramagnetic rare earth and a transition metal may feature complex magnetic behavior as the moments on these atoms couple in various ways at different temperatures, as is seen for compounds such as SmCo5 and PrCo2P2 [68,69]. Cerium-containing intermetallics can exhibit particularly interesting phenomena due to the fact that the energy of the Ce 4f electron is often close to the Fermi level, leading to the possibility of two valence states (Ce3+ or Ce4+) and variation in valence with temperature or pressure. This can result in mixed valence structures

(Ce23Ru7Cd4) [70], valence fluctuation (Ce2RuZn4) [71], and heavy fermion behavior (CeRhIn5) [72]. In this work, two structurally related cerium iron borocarbide intermetallics have been grown from Ce/T eutectic fluxes. Both structures feature several cerium sites surrounding clusters of iron capped with borocarbide chains. Ce33Fe13.1Al1.1B24.8C34 exhibits cerium valence

49 fluctuation; its boron-centered Fe14 clusters do not possess magnetic moments. Ce33Fe13B18C34 is comprised of tetravalent Ce ions (possibly the result of chemical pressure) and cuboctahedral

Fe13 clusters which order ferromagnetically.

The Fe13 and Fe14 clusters are highly unusual structural building blocks. R33Fe14- xAlx+yB25-yC34 (R = La, Ce) and R33Fe13-xAlxB18C34 (R = Ce, Pr) occupy the relatively unexplored phase space between intermetallic compounds with isolated (and usually non-magnetic) iron sites such as La3.67FeC6 and ErFe4Al9Si6, and compounds with extended iron building blocks featuring strong magnetic moments, such as R6Fe13-xAlx and Nd2Fe14B [73, 74, 75, 76]. The variation in magnetic properties of the iron clusters in the title phases is due to different Fe-Fe distances and number of nearest neighbor iron atoms, as well as the extent of aluminum substitution on iron sites. Tailoring magnetic properties by careful control of substitution in phases such as Nd2Fe14-xMxB and SmCo5-xMx is a topic of widespread interest; [76] studying this process in the smaller iron clusters available in these new Ce/Fe/B/C phases may shed light on how magnetic moments develop in hard magnetic materials.

5.2 Materials and Methods 5.2.1 Synthesis Starting materials were stored and handled in an argon-filled dry-box. Powders of the following elements were used in reactions: carbon black (Strem Chemicals 95-97%), boron (Strem Chemicals 95-97%), iron (Alfa Aesar 99%), manganese (Alfa Aesar 99%), aluminum (Alfa Aesar 99%), nickel (Alfa Aesar 99%), and iron-57 (Isoflex USA 96.63% enrichment).

Iron-57 was used for the synthesis of an enriched sample of Ce33Fe14-xAlx+yB25-yC34 for Mössbauer spectroscopy. R-T eutectics were either purchased (La-Ni, Alfa-Aesar; 67% La with mp 517°C) or made by arc-melting appropriate amounts of rare earth and transition metals under argon. For instance, Ce-Co eutectic was made from cerium ingot (Alfa Aesar, 99.999%) and cobalt slug (Alfa Aesar 99.95%), by arc melting a 76%:24% mole ratio of these elements (m.p. 424°C) [6] on a water-cooled copper hearth into a button that was turned over and re-melted several times to ensure homogeneity of the flux material. Similarly, Ce-Fe eutectic was made from cerium ingot and iron slug (Alfa Aesar 99%) in a 83%:17% mole ratio (m.p. 592°C); Ce- Ni eutectic from cerium ingot and nickel slug (Alfa Aesar 99%) in a 78%:22% mole ratio (m.p. 477°C); Pr-Fe eutectic from praseodymium ingot (Alfa Aesar 99.9%) and Fe slug using a

79%:21% mole ratio (m.p. 620°C); Pr-Ni eutectic from praseodymium ingot and nickel slug in a 81%:19% mole ratio (m.p. 460°C) [6]. These brittle eutectic ingots were fragmented into approximately 1 mm3 size pieces; 1.5 g of eutectic were used per reaction. Reactions in the various R-T eutectics were prepared by sandwiching C, B, Ni, Al, and either Mn, Fe or Fe-57 (combined in a 1:1:1:1:1 mmole ratio) between layers of R-T eutectic, with more flux on the top layer than the bottom one. Reactants and flux were loaded into either alumina or steel crucibles (ca. 8 mm inside diameter, 30 mm length) and a second alumina crucible filled with Fiberfrax was inverted and placed on top of the reaction crucible to act as a filter during centrifugation. These crucible configurations were placed into silica tubes, and fused under a vacuum of 10-2 Torr. The ampoules were then heated to 950°C in 3 hours, held at this temperature for 12 hours, and then cooled to 850°C in 10 hours. The reaction mixtures were subsequently annealed for 48 hours at 850°C and then cooled to 600°C in 84 hours. At 600°C the ampoules were removed from the furnace, quickly inverted, and placed into a centrifuge to decant the molten flux. (Reactions in the higher-melting Ce-Fe eutectic were instead cooled to 700°C and centrifuged.) Products from these reactions were stored in an argon-filled dry-box in order to prevent oxidation.

Stoichiometric syntheses of R33Fe14-xAlx+yB25-yC34 and R33Fe13-xAlxB18C34 were attempted by combining elemental forms of R (R = La, Ce, Pr, Nd, Sm), Fe, B, and C in appropriate ratios (for instance, a 6.6:2.8:5.0:6.8 mmol ratio for R33Fe14B25C34) in an alumina crucible. These reactions were prepared in an argon-filled dry-box and the crucible was placed in a fused silica tube and sealed under a vacuum of 10-2 Torr. The ampoule was then heated to 950°C in 3 hours, held at this temperature for 168 hours, and then cooled to 25°C in 3 hours. Products were stored in an argon-filled dry-box to prevent oxidation of the powders.

5.2.2 Elemental Analysis Elemental analysis was performed on all samples using a JEOL 5900 scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDXS) capabilities. Flux-grown crystals from each reaction were affixed to an aluminum SEM puck using carbon tape, fractured to expose inner regions, and positioned so that flat faces were perpendicular to the electron beam. Samples were analyzed using a 30kV accelerating voltage and an accumulation time of

60s. A rare earth:iron ratio of 7:3 was consistently found for R33Fe14-xAlx+yB25-yC34 and R33Fe13-

51 xAlxB18C34 phases; nickel content was negligible (below 1 mol%). Small amounts of aluminum

(1-2%) were indicated in the R33Fe14-xAlx+yB25-yC34 and Pr33Fe13-xAlxB18C34 crystals. No aluminum was indicated in the data for Ce33Fe13B18C34 samples. The carbon and boron content was not able to be determined due to the limitation of the EDXS in detecting the characteristic

X-rays of light elements. Quantitative determination of the carbon content of Ce33Fe14-xAlx+yB25- yC34 was carried out by combustion analysis (Atlantic Microlabs). A 60 mg sample of Ce33Fe14- xAlx+yB25-yC34 crystals was sent for analysis to determine the amount of C present. Two runs of 30 mg each were performed, yielding carbon mass percents of 5.46% and 5.57%.

5.2.3 X-ray Photoelectron Spectroscopy

Analyses of cerium valence and light elements were performed on Ce33Fe14-xAlx+yB25- yC34 and Ce33Fe13B18C34 samples previously screened by EDS, using a Physical Electronics PHI 5100 series XPS with a non-monochromated dual anode (Al & Mg) source having a single channel hemispherical energy analyzer. Large single crystals were affixed to a carbon coated sample puck using carbon tape. The Al X-ray source was used to study higher binding energies. To remove surface oxides and flux residue, samples were sputtered by Ar+ ions as the sample stage was rotated. Spectra were taken after every 5-10 minutes of sputtering to monitor the disappearance of surface oxide peaks and appearance of new species. Once no more changes in spectra were observed, sputtering was stopped.

5.2.4 X-ray diffraction Selected single crystals of each phase were mounted on cryo loops using paratone oil. X- ray diffraction data were collected at 150K on a Bruker SMART APEX2 CCD diffractometer equipped with a Mo-target X-ray tube (λ = 0.71073). The data sets were recorded as ω scans at 0.3° stepwidth and integrated with the Bruker SAINT software. Data were corrected for absorption effects using the multiscan method (SADABS) [43]. The structures were refined in the centrosymmetric space group Im-3m (space group number 229) by full matrix least squares procedures on |F2| using the SHELX-97 software package [44]. The rare earth and iron atoms were located using direct methods, and the light atoms were indicated by residual electron density peaks of 5 – 10 e-/Å3 in the difference Fourier maps. Light atom sites were initially assigned as boron. Allowing their occupancy to vary did not distinguish boron from carbon (the

scattering factors of these elements are too similar), so assignments of these sites were based on comparison of observed bondlengths to those in other rare earth borocarbides (see discussion). During final refinement cycles occupancies of each site were allowed to vary to identify possible site mixing. Unit cell parameters and crystallographic data collection information are found in table 5.1 and atomic coordinates and displacement parameters are found in table 5.2. Selected bond lengths are found in table 5.3.

Table 5.1 Single crystal collection information for selected samples

Ce33Fe13.1Al1.1B24.8C34 Ce33Fe13B18C34 Pr33Fe12.9Al0.1B18C34 Formula weight 6.055.58 5952.93 5977.8 (g/mol) Crystal System Cubic Space group Im-3m a, Å 14.617(1) 14.246(8) 14.3881(9) Z 2 2 2 Volume 3123.2(4) 2891(3) 2978.6(3) Density, calc (g/cm3) 6.44 6.84 6.67 Radiation Mo Kα Temperature (K) 150 Index Ranges -18≤h≤19 -18≤h≤18 -18≤h≤19 -19≤k≤19 -18≤k≤18 -19≤k≤18 -19≤l≤19 -18≤l≤18 -18≤l≤19 Theta range (°) 1.97 to 25.21 2.02 to 28.25 2.00 to 28.20 Reflections collected 17445 15157 16857 Unique 419/40 388/36 402/38 data/parameters µ (mm-1) 26.41 28.51 29.44 R1/wR2 0.0155/0.0350 0.0142/0.0285 0.0127/0.0271 R1/wR2 (all data) 0.0186/0.0354 0.0143/0.0285 0.0137/0.0273 Residual Peak/hole 1.015/-1.350 0.826/-0.679 0.834/-1.430 (e-Å-3)

Table 5.2 Atomic positions and displacement parameters for select R/T/B/C phases.

Ce33Fe13.1Al1.1B24.8C34 Atom Wyckoff Occupancy x y z Ueq site Ce1 48k 1 0.12831(1) 0.12831(1) 0.30862(2) 0.0073(1) Ce2 12d 1 ¼ 0 ½ 0.0054(1) Ce3 6b 1 0 ½ ½ 0.0054(2) Fe1 16f 1 0.08812(5) 0.08812(5) 0.08812(5) 0.0046(3)

Fe2/Al2 12e 0.85(1)/0.15(1) 0.2015(1) 0 0 0.0056(5) B1 24h 1 0 0.1643(4) 0.1643(4) 0.007(1) B2 24h 1 0 0.3071(4) 0.3071(4) 0.008(1) B3/Al3 2a 0.79(5)/0.21(5) 0 0 0 0.006(6) C1 24h 1 0 0.2372(4) 0.2372(4) 0.007(1) C2 24h 1 0 0.3781(4) 0.3781(4) 0.011(1) C3 12e 1 0.333(1) 0 0 0.035(3) C4 8c 1 ¼ ¼ ¼ 0.010(2)

Ce33Fe13B18C34 Atom Wyckoff Occupancy x y z Ueq site Ce1 48k 1 0.12638(1) 0.12638(1) 0.30929(2) 0.0054(1) Ce2 12d 1 ¼ 0 ½ 0.0057(1) Ce3 6b 1 0 ½ ½ 0.0043(1) Fe1 24h 1 0 0.12800(5) 0.12800(5) 0.0056(2) Fe2 2ª 1 0 0 0 0.0130(8) C1 24h 1 0 0.2246(4) 0.2246(4) 0.008(1) C2 24h 1 0 0.3735(4) 0.3735(4) 0.009(1) C3 12e 1 0.3297(9) 0 0 0.017(2) C4 8c 1 ¼ ¼ ¼ 0.011(3) B1 24h 1 0 0.2991(4) 0.2991(4) 0.005(1) B2 12e 1 0.2188(9) 0 0 0.008(2)

Table 5.2 Continued: Atomic positions and displacement parameters for select R/T/B/C phases.

Pr33Fe13B18C34 Atom Wyckoff Occupancy x y z Ueq site Pr1 48k 1 0.12634(1) 0.12634(1) 0.30713(2) 0.0071(1) Pr2 12d 1 ¼ 0 ½ 0.0057(1) Pr3 6b 1 0 ½ ½ 0.0056(1) Fe1 24h 0.96(3)/0.04(3) 0 0.12765(5) 0.12765(5) 0.0071(2) Fe2 2ª 1 0 0 0 0.009(1) C1 24h 1 0 0.2248(3) 0.2248(3) 0.008(1) C2 24h 1 0 0.3707(3) 0.3707(3) 0.011(1) C3 12e 1 0.328(1) 0 0 0.043(3) C4 8c 1 ¼ ¼ ¼ 0.019(3) B1 24h 1 0 0.2970(4) 0.2970(4) 0.005(1) B2 12e 1 0.2119(9) 0 0 0.016(2)

Table 5.3 Bond lengths (in Å) for R/T/B/C phases

Ce33Fe13.11(1)Al0.89(1)B25C34 Ce33Fe13B18C34 Pr33Fe13B18C34 Ce1-C4 2.6576(4) R1-C4 2.630(1) 2.6471(3) Ce1-C1 2.672(1) R1-C1 2.580(1) 2.5912(5) Ce1-B1 2.871(3) R1-B1 3.053(5) 3.058(4) Ce1-Ce1 3.7274(6) R1-R1 3.601(2) 3.6356(4) Ce1-Ce2 3.8089(4) Ce2-Ce3 3.6543(3) R2-R3 3.562(1) 3.5970(2) B1-C1 1.507(11) B1-C1 1.501(11) 1.469(10) B1-Fe1 2.035(6) C2-B1 1.499(11) 1.500(10) B1-Fe2 2.462(4) C3-B2 1.579(19) 1.667(21) C1-B2 1.445(11) R1-B2 2.854(6) 2.913(6) B2-Ce2 2.941(4) B2-Ce1 3.100(4) R1-C3 2.563(2) 2.588(2) C2-B2 1.469(11) R1-Fe1 3.148(2) 3.1581(6) C2-Ce3 2.519(8) R2-C2 2.519(1) 2.5450(3) C2-Ce2 2.5848(3) R2-B1 2.946(5) 2.998(4) C2-Ce1 2.8007(5) Fe1-B3 2.231(1) Fe1-B2 2.236(8) 2.200(8) Fe1-Fe2 2.462(1) Fe1-Fe1 2.579(2) 2.598(1) Fe1-Fe1 2.576(1) Fe1-Fe2 2.579(2) 2.598(1)

Powder X-ray diffraction data were collected using a Rigaku Ultima III diffractometer with a Cu radiation source and a CCD detector. Samples were ground with a small amount of silicon to act as an internal standard. Experimental powder patterns were compared to calculated patterns based on single crystal data to confirm the presence of the desired phase (figure 5.1) Unit cell parameters were determined by using the accompanying refinement software contained in the JADE software (a = 14.850(9)Å, 14.581(4)Å, and 14.390(1)Å for La33Fe14-xAlx+yB25-yC34 ,

Ce33Fe14-xAlx+yB25-yC34, and Pr33Fe13-xAlxB18C34, respectively). Byproducts were identified by comparison with patterns in the JADE software database.

Ce33Fe13B18C34 calculated Ce33Fe14B25C34 calculated Stoichiometric synthesis Ce33Fe13B18C34 Stoichiometric synthesis Ce33Fe14B25C34 Stoichiometric synthesis Pr Stoichiometric Synthesis Nd Stoichiometric synthesis Sm SDT data

20 25 30 35 40 45 50 55 60 2-Theta

Figure 5.1 Powder diffraction data for products of R/Fe/B/C reactions synthesized using stoichiometric mixtures of elements in alumina crucibles, compared to the theoretical patterns for Ce33Fe13B18C34 and Ce33Fe14B25C34 (bottom two patterns) calculated from the structural parameters derived from single crystal XRD studies. Stoichiometric syntheses with cerium produce only the Ce33Fe14B25C34 phase. Attempts to make analogs of either structure with Nd or Sm produce only binary or pseudobinary phases.

5.2.5 Magnetic Susceptibility Magnetic measurements were carried out with a Quantum Design MPMS SQUID magnetometer. For each sample, a large single crystal was weighed and then placed at the 3 cm mark of a 12 cm piece of kapton tape. The kapton tape was folded in half to enclose the crystal. Then the piece of tape containing the crystal was placed in a plastic straw and then loaded into the magnetometer. Both field-cooled (FC) and zero-field cooled (ZFC) measurements were collected at an applied field of 100 Oe with a temperature range of 2 – 300 K, and field dependence data were collected from 0 to 70,000 Oe at several temperatures. Magnetic measurements were carried out with a Quantum Design MPMS SQUID magnetometer. For each sample, a large single crystal was weighed and then placed at the 3 cm mark of a 12 cm piece of kapton tape. The kapton tape was folded in half to enclose the crystal. Then the piece of tape containing the crystal was placed in a plastic straw and then loaded into the magnetometer. Both field-cooled (FC) and zero-field cooled (ZFC) measurements were collected at an applied field of 100 Oe with a temperature range of 2 – 300 K, and field dependence data were collected from 0 to 70,000 Oe at several temperatures.

5.2.6 Mössbauer Spectroscopy

Mössbauer Spectroscopy measurements on Ce33Fe14-xAlx+yB25-yC34 grown using enriched 57Fe were carried out using a SEE Co. Mössbauer spectrometer installed with a gamma source, 57CO embedded in Rh matrix, with an initial intensity of 100 milliCurie manufactured by Isotope. Temperature was controlled with a Janis helium cryostat. Measurements were performed at ±3 mm/s at 5.5, 78, 150, and 294K, and at ±10 mm/s at 5.5 and 78 K.

5.2.7 Solid State NMR spectroscopy 11 B MAS-NMR spectra of Ce33Fe14-xAlx+yB25-yC34 were collected on a Varian Unity Inova 500MHz wide-bore NMR spectrometer equipped with a three channel MAS probe. The 11 Larmor frequency of B was 160.538 MHz. Single crystals of Ce33Fe14-xAlx+yB25-yC34 were ground with NaCl in a dry box (2:1 sample to NaCl by mass) to facilitate spinning of the sample in the magnetic field; the mixture was packed into a 4 mm zirconia rotor sealed with airtight screw caps. A typical small angle pulse (less than 15°) was applied to ensure quantitative spectra with a recycle delay of 5s. Data were collected at 25°C and a 14 kHz spin rate was used. A total of 14920 scans were collected for the NMR spectrum. The spectra were referenced to solid 57

13 NaBH4 at -42.16 ppm (with respect to BF3OEt2 at 0 ppm). Attempts were made to obtain C MAS NMR data but no resonances were observed.

5.2.8 Resistivity Measurements

Resistivity measurements were conducted on Ce33Fe14-xAlx+yB25-yC34 by a conventional four-point dc method on a Physical Property Measurement System (PPMS) by Quantum Design. A crystal (size: 1mm x 1mm x 1mm) was mounted on the sample holder of a 4He probe with a small amount of glue and connected to the electrodes of the sample holder with 0.001 in. diameter gold wires using silver paint. Measurements were carried out from 1.8−300 K, using an applied excitation current of 3 mA.

5.3 Results and Discussion All of the binary alloy phase diagrams of early rare earth elements (R = La, Ce, Pr, Nd) combined with late first row transition metals (T = Fe, Co, Ni, Cu) contain a low-melting eutectic in the rare-earth rich region [6]. These eutectic melts have proven to be excellent solvents for iron, carbon, and most other elements, leading to formation of many new R/Fe/C/X phases. A recent review of known ternary R/T/C carbides classified such compounds into two groups: carbometallates (such as La3.67FeC6, featuring isolated transition metal atoms surrounded by carbon), or transition metal-rich carbides (such as Nd2Fe17C3-x, featuring extensive Fe-Fe bonding)[8]. However, several new phases have recently been discovered which fit between these classifications. Compounds such as La21Fe8Sn7C12 (grown in La/Ni flux) [67],

Dy15Fe8C25[77], and the title phases R33Fe14-xAlx+yB25-yC34 (R = La, Ce) and R33Fe13-xAlxB18C34 (R = Ce, Pr) feature iron clusters of 4 to 14 atoms capped around the edges by carbon and/or boron. The intermediate size of these iron building blocks can lead to complex magnetic behavior. This complexity is furthered in the cerium iron borocarbide title phases due to the variable valency of cerium.

5.3.1 Synthesis

R33Fe14-xAlx+yB25-yC34 (R = La, Ce) and R33Fe13-xAlxB18C34 (R = Ce, Pr) grow from various R-T eutectic fluxes as large reflective, silver facetted spheroids up to 1-2 mm in diameter; see Figure 5.2.

Figure 5.2 SEM image of crystals of Ce33Fe14B25C34 grown from Ce/Fe flux, displaying truncated octahedral growth habit.

A manganese analog of one of the structures, Ce33Mn14Al0.1B24.9C34, can also be synthesized. The compounds degrade slowly in air (over several days) and react rapidly with water. While product can be obtained from all the fluxes that were investigated, the highest yields were found for reactions in Ce-Co eutectic for Ce33Fe14-xAlx+yB25-yC34 (70% yield based on carbon) and Pr-Ni flux for Pr33Fe13-xAlxB18C34 (40% yield based on carbon; yield was significantly lower in Pr-Co). Yield increased with the addition of C and B, up to 1.2 mmol of each element added. Beyond this amount, unreacted C and B were observed in the centrifuged ampoules, indicating a possible solubility limit. The Ce-Co and Ce-Ni fluxes are more convenient to use compared to the higher melting Ce-Fe eutectic, although they present the risk of cobalt or nickel incorporation into products. These elements were not observed in the EDS analyses of Ce33Fe14-xAlx+yB25-yC34, but trace amounts of cobalt were detected in the manganese analog Ce33Mn14Al0.1B24.9C34 grown from reactions in Ce-Co flux. This parallels what has been observed in previous reactions in La/Ni flux. For instance, in La/Ni flux syntheses of La6T13- xAlx, reactions with T = Fe yield La6Fe10Al3 with no nickel incorporated; if T = Mn is used instead, the La6Mn10Al3 products do show doping of small amounts of nickel on some of the manganese sites [46]. The reactivity of transition metals in these R/T eutectics—whether as added reactants or as components of the flux eutectic—appears to be Fe > Mn >> Co, Ni. The synthesis of the title phases is complicated by the fact that additional elements (Al and Ni) must be present in the flux to aid in their growth. A series of reactions were carried out to determine which elements were necessary for product formation. Reactions that did not 59 contain both Ni and Al yielded predominantly pseudo-binary phases such as CeFe2-xCox. Incorporation of trace amounts of aluminum may be required to stabilize the structures. This is also seen for phases such as LaFe13-xAlx and La6Fe13-xAlx; the binaries LaFe13 and La6Fe13 are not known [46,78] EDS analyses and structural refinement of single crystal XRD data for several “R33Fe14B25C34” crystals consistently indicates substitution of Al on the Fe2 site and the

B3 site, resulting in a stoichiometry of R33Fe14-xAlx+yB25-yC34 (see Table 1). These phases were initially grown in alumina crucibles without Al reactant; the strongly reducing flux reacted with the crucible, adding aluminum metal to the reaction. When aluminum was deliberately added to subsequent reactions, the yield increased drastically. A very small amount of aluminum substitution is also seen in Pr33Fe13-xAlxB18C34 (x ≈ 0.1), but is not seen for the Ce33Fe13B18C34 analog. Nickel does not appear to be incorporated into the products at all, so its role in the reaction may be to aid in solubilizing the boron reactant or to catalyze or nucleate the crystal growth of the phase. Similar behavior was observed for the gallium flux syntheses of

Yb3Ga7Ge3 and β-SiB3, which require the presence of Pd and Cu respectively[79,80]. Reactions that did not include Ni resulted in a very small yield of product and unreacted boron. Attempts at using copper or other elements instead of nickel resulted in unreacted boron and crystals of

CeFe2-xCox instead of the title phases. Despite the complexity of the synthesis, several analogs of each structure can be obtained reproducibly.

The identity of the rare earth determines whether R33Fe14-xAlx+yB25-yC34 or R33Fe13- 3+ 3+ xAlxB18C34 is formed, with larger La ions producing the former structure, and smaller Pr ions yielding the latter. This is confirmed in both flux and stoichiometric synthesis. La33Fe14- xAlx+yB25-yC34 is formed from either La/T flux reactions (as crystals) or as silver powder from stoichiometric combination of the elements. Pr33Fe13-xAlxB18C34 is likewise isolated as crystals from Pr/T fluxes or as silver powder in stoichiometric reactions. Cerium ions can form both structures. For Ce flux reactions, the formation of either Ce33Fe14-xAlx+yB25-yC34 or

Ce33Fe13B18C34 is determined by the crucible used for synthesis. For reactions carried out in alumina crucibles, Ce33Fe14-xAlxB25C34 is formed, possibly induced by leaching of trace Al from the crucible. Reactions carried out in steel crucibles produce Ce33Fe13B18C34 instead. Trace elements in the steel may promote the formation of this structure. Attempts at stoichiometric synthesis of the cerium phases produce only the Ce33Fe14-xAlx+yB25-yC34 phase. The powder

pattern matches that calculated from the single crystal data for this structure (Figure 5.1). The amount of Al incorporation was unable to be determined because no single crystals were obtained, although trace amounts of Al were indicated in the EDS analysis of the powder. Flux

reactions and stoichiometric syntheses using Nd and Sm did not form either R33Fe14-xAlx+yB25-

yC34 or R33Fe13-xAlxB18C34 phases; these elements are too small to occupy the rare earth sites in these structures. Light elements such as boron and carbon cannot be analyzed by SEM-EDXS; large

crystals of Ce33Fe14-xAlx+yB25-yC34 were therefore studied using XPS to determine whether or not these elements were present. Ar+ ion sputtering was used to remove surface species from samples before XPS measurement. No boron peaks were observed before sputtering, possibly indicating coating of the crystals by traces of flux or surface enrichment by other elements. After sputtering, a peak appears at 196 eV, which is in the expected B 1s binding energy region [25]. The presence of carbon was also confirmed by the XPS data with the C 1s binding energy

of 284 eV similar to that of other carbides such as Fe3C (283.9 eV), SiC (283.8 eV), and BaC2

(283.5 eV) [25]. Carbon content was quantified by combustion analysis on samples of Ce33Fe14-

xAlx+yB25-yC34 (Atlantic Microlabs), which indicated 5.5% carbon by mass. This is lower than the 6.7% value expected from the stoichiometry, which may be due to flux coating of the samples, or oxidation during handling.

5.3.2 Crystal Structure of R33M14-xAlx+yB25-yC34 (R = La, Ce; M = Fe, Mn) This structure crystallizes in the cubic space group Im-3m with unit cell parameters decreasing with the size of the rare earth (Table 5.4).

Table 5.4 Unit cell parameters of all structures.

Compound Unit cell (Å) r-factor (R1/wR2)

La33Fe13.2(1)Al0.8(1)B25C34 14.88(1) 0.0165 / 0.0317

Ce33Fe13.11(1)Al0.89(1)B25C34 14.617(1) 0.0157 / 0.0350

Ce33Mn14B25C34 14.615(1) 0.0154 / 0.0343

Ce33Fe13B18C34 14.274(4) 0.0177 / 0.0306

Pr33Fe13B18C34 14.4220(6) 0.0166 / 0.0450

La33Fe13.2(1)Al0.8(1)B25C34 14.88(1) 0.0165 / 0.0317

The structure can be viewed as a bcc packing of borocarbide-capped Fe14B clusters as shown in Figure 5.3. The reaction of iron with lightweight metals or metalloids in R/T fluxes to form iron clusters or layers capped by the light elements is a reoccurring theme. In addition to the two types of iron clusters reported here, carbon-capped tetrahedral Fe4 units are found in the

La21Fe8M7C12 structure [61], and aluminum-capped iron layers in La6Fe13-xAlx, both grown in

La/Ni flux [46]. The iron cluster in R33Fe14-xAlx+yB25-yC34 is a face capped cube (or tetrakis hexahedron) of 14 iron atoms, with six Fe2 atoms (12e sites) at the capping apices and eight Fe1 sites (16f sites) defining the cube. The atom that centers this cluster (on the 2a site) is much closer to the Fe1 sites than the Fe2 sites (2.231(1)Å vs 2.946(1)Å in the Ce33Fe14-xAlx+yB25-yC34 analog). The mall electron density at this site indicated that it is occupied by a light atom, either B or C. The 2.231(1)Å distance from this site to the Fe1 atoms is much longer than typical Fe-C bonds in intermetallics (which are generally 2.0Å or shorter) [61,81]. It is only slightly longer than the Fe-B bond lengths of 2.118 Å and 2.180 Å found in phases such as Nd2Fe14B and FeB

[78,82]. Boron is therefore a more likely option. In the Ce33Mn14B25C34 analog, the corresponding Mn-B bond length of 2.203(1)Å is within the range observed in MnB4 (2.049- 2.218 Å), further supporting the assignment of this site as boron [83]. For all analogs, allowing the occupancy to vary resulted in a higher than 100% occupancy. This, and the slightly long Fe- B bondlengths, points to incorporation of a larger, heavier element on this 2a site. It was therefore modeled as a mixture of boron and aluminum; this improved the refinement (yielding more stable thermal parameters) and indicated incorporation of 10-20% Al on this site.

Figure 5.3 (a) Structure of Ce33Fe13.1Al1.1B24.8C34. Red spheres are iron, green spheres are cerium, black spheres are carbon, blue spheres are boron. The boron-centered Fe14 clusters are drawn as red polyhedra and the isolated carbon anion in octahedral coordination as grey polyhedra. (b) Borocarbide-capped Fe14 cluster and associated bondlengths. (c) Ce1 coordination environment (48k site). (d) Ce2 coordination environment (12d site). (e) Ce3 coordination environment (6b site). (f) Coordination environment around the borocarbide chain.

The Fe14B cluster in Ce33Fe14-xAlx+yB25-yC34 features Fe-Fe bond lengths of 2.462(1) Å and 2.576(1) Å. These are similar to the 2.55Å bondlength in the Fe4 clusters of La21Fe8Sn7C12 and well within the range of 2.429 Å to 2.740 Å observed for Ce2Fe17 [61,84]. In the manganese analog, the Mn-Mn bond lengths (2.462(1) and 2.544(1) Å) in the Mn14B cluster compare well with those found in β-Mn which range from 2.363 Å-2.680 Å [85]. A small amount of Al substitution (15 %) is observed for the Fe2 sites of Ce33Fe14-xAlx+yB25-yC34. No Al substitution on the Mn sites was observed based on the refinement of the occupancies of these atoms from the single crystal data of Ce33Mn14B25C34, but trace amounts of Co observed in the EDXS analysis suggests there may be incorporation of cobalt from the flux on these sites. The iron clusters are capped at the six vertices by carbon atoms, and capped on 12 edges by 4-atom linear chains of carbon and boron (Figure 5.3b). The title compounds thus add to the small database of reported structures containing borocarbide chains, which includes R10B9C12 (R

= La, Ce, Pr, Nd), R5B2C5 (R = Ce-Tm), R5B4C5 (R = Ce, Pr, Nd), RBC (R =Ce, Pr, Nd),

R15B6C20, CeB2C2, Lu3BC3, and Gd4B3C4 [86, 87, 88, 16, 89, 90, 91, 92]. Assignments of the carbon and boron sites in these chains were made based on bond lengths within the chain and distances to cerium atoms surrounding this chain, and comparison to other metal borocarbide structures. The B1 site caps an edge of the Fe14B cluster; it is bonded to 2 Fe atoms, 1 C atom, and 4 Ce atoms. The associated bond lengths are shown in Figure 5.3b and listed in Table 5.3.

These bond lengths compare well with those reported for similar compounds such as CeB2C2 with a Ce-B bond distance of 2.8402(2) Å, and B-C bond distances of 1.5312(9) Å to 1.6220(6)

Å [90]. The rest of the Ce33Fe14-xAlx+yB25-yC34 borocarbide chain extends from the B1 atom, with bond distances between the C1-B2-C2 atoms of C1-B2 of 1.45(1) Å and B2-C2 of 1.47(1) Å. These are similar to the 1.44 Å to 1.48 Å range of bond lengths observed in other compounds containing linear C-B-C units such as Gd4B3C4 and Lu3BC3 [91,92]. Termination of these chains by C atoms is thought to be favored due to the surrounding octahedral coordination environment, and on the basis of the higher electronegativity of carbon and its affinity for the electropositive rare earth ions [93]. The distances from atoms in the borocarbide chain of

Ce33Fe14-xAlx+yB25-yC34 to the surrounding cerium ions exhibit the expected trend of shorter Ce-C bonds (all below 2.8005(6)Å) and longer Ce-B bonds (all above 2.871(3)Å); see Figure 2f.

The R33Fe14-xAlx+yB25-yC34 structure also features an isolated carbon site (C4) surrounded by six cerium cations in an octahedral configuration. These carbide anions (presumably C4-) and their coordination environment are depicted as grey octahedra in Figure 5.3. The six R-C bondlengths are all equal and are only slightly affected by replacing Fe with Mn in the structure (Ce-C distances 2.6576(4) Å and 2.657(2)Å for tfhe Fe and Mn analogs, respectively).

5.3.3 Crystal Structure of R33M13-xB18C34 (R = Ce,Pr; M = Fe, Mn)

R33Fe13-xAlxB18C34 crystallizes in the centrosymmetric space group Im-3m, with unit cell parameters of a = 14.246(8) Å for the Ce analog and a = 14.3881(9) Å for the Pr analog. The

Fe13 iron cluster in this structure is perfectly cuboctahedral and is centered by another iron atom instead of a boron atom (Figure 5.4). The Fe-Fe bond lengths between neighboring iron atoms and those to the center iron are all equal; this distance of 2.579(2) Å for the cerium analog is slightly longer than the distances seen for Ce33Fe14-xAlx+yB25-yC34 and La21Fe8Sn7C12, but is well within the ranges seen in Ce2Fe17 and α-Fe (2.482 Å-2.866 Å) [61,84]. A very small amount of

64 aluminum substitution (4%) is seen on the central Fe2 site for Pr33Fe12.9Al0.1B18C34, but not for the cerium analogue. All twelve of the Fe1 atoms defining the cuboctahedron are capped by a C- B-C chain. The Fe1-C1 bond is 1.947(8) Å in length; subsequent bond lengths in the chain are C1-B1 of 1.50(1) Å and B1-C2 of 1.50(1) Å, comparing well to those in other structures. The six square faces of the cuboctahedron are each capped by a B-C unit with a Fe1-B2 distance of

2.236(8)Å, very similar to the distances between the Fe14B cluster to the capping BCBC chains in Ce33Fe14-xAlx+yB25-yC34. The B2-C3 distance of 1.58(2)Å is somewhat longer than expected, but the distances from the C3 site to surrounding rare earth ions support the assignment of this site as carbon and not boron. All terminal C atoms are bonded to Ce atoms at distances of 2.43(1) Å - 2.717(1) Å, forming a framework that surrounds the central Fe cluster. As is seen in 4- the R33Fe14-xAlx+yB25-yC34 structure, R33Fe13-xAlxB18C34 also features isolated C anions (C4 sites) octahedrally coordinated by R cations with six equivalent C-Ce distances of 2.630(1)Å.

It is notable that Ce33Fe13B18C34 has a smaller unit cell than its praseodymium analog. All bondlengths associated with rare earth sites are shorter in the Ce phase compared to the Pr analog (see Table 3), contrary to what is expected given the relative sizes of Ce3+ and Pr3+. The

Ce-X bonds are also shorter than similar bonds in Ce33Fe14-xAlx+yB25-yC34, with Ce-Ce bonds in particular shrinking from 3.6543(3) Å and 3.7274(6) Å in the latter to 3.562(1)Å and 3.601(2)Å 4+ in Ce33Fe13B18C34. This indicates a high likelihood of the presence of Ce in Ce33Fe13B18C34.

To confirm this, XPS data were collected on crystals of Ce33Fe13B18C34. After sputtering to remove surface oxide species, the spectrum features two broad peaks at binding energies of 900 and 918 eV (Figure 5.12), with associated shake-down peaks at 895 and 913 eV. These 4+ correspond to a 3d5/2-3d3/2 spin-orbit doublet which arises exclusively from Ce ; the 918 eV peak in particular does not overlap with any possible transitions characteristic of Ce3+ and is viewed as strongly indicative of tetravalent cerium [94,95].

Figure 5.4 (a) Structure of Ce33Fe13B18C34. Red spheres are iron, green spheres are cerium, black spheres are carbon, blue spheres are boron. The cuboctahedral Fe13 clusters are drawn as red polyhedra and the isolated carbon anion in octahedral coordination as grey polyhedra. (b) Fe13 cluster and associated bondlengths. (c) Ce1 coordination environment (48k site). (d) Ce2 coordination environment (12d site). (e) Ce3 coordination environment (6b site). (f) Coordination environment around the borocarbide chain.

The formation of tetravalent cerium in this compound may be induced by chemical 3+ pressure. The R33Fe13B18C34 structure is stable for the smaller rare earth cation Pr ; the packing 3+ of the Fe13 clusters and associated borocarbide chains promotes the conversion of Ce cations to 4+ smaller Ce ions to stabilize the compound. Similar effects are seen for systems such as CexY1- 3+ xAl3, where substitution of smaller Y into the rare earth sites causes the average surrounding coordination environment to shrink and induces formation of Ce4+ ions [96].

5.3.4 Transport properties of Ce33Fe14-xAlx+yB25-yC34

Resistivity measurements performed on a single crystal of Ce33Fe14-xAlx+yB25-yC34 indicate that this material behaves as a poor metal (Figure 5.5). The room temperature resistivity is 5.39 Ω·cm; this lies in the overlap of typical metallic (10−5−101 Ω·cm) and semiconductor (10−2−105 Ω·cm) resistivity ranges [26]. The resistivity rises as the temperature is increased, as is expected for metals. Two kinks in the temperature dependence are observed, one at low

66 temperature (10 K) which corresponds to a magnetic ordering transition, and one at 125 K which may be due to cerium fluctuating valence; both these features are mirrored in the magnetic susceptibility data (vide infra).

Figure 5.5 Resistivity data for Ce33Fe13.1Al1.1B24.8C34, measured on a single crystal.

NMR studies were carried out on Ce33Fe14-xAlx+yB25-yC34 in an attempt to glean information about boron and carbon siting as well as the interaction of these nuclei with conduction electrons. While no 13C resonances could be observed (likely due to low natural abundance and broadening from surrounding paramagnetic atoms), the 11B MAS NMR spectrum shows a single peak at 11.2 ppm (Figure 5.5). This is likely due to the B2 site, which is bonded to two carbon atoms; the resonances of the two other boron sites (both bonded directly to iron) 11 may be too broadened to be observed. This B shift is similar to those reported for YB4, LaB4 and CaCxB4-x, all of which have resonances in the 5 to 56 ppm range[97,98]. In these phases, the small shift (relative to the larger Knight shifts expected for metals) is attributed to a small boron s-electron density of states at the Fermi level. Polar intermetallic phases often feature a pseudogap in the DOS at Ef,[99] which can lead to poor conductivity and a small Knight shift.

The NMR data and resistivity measurements suggest that Ce33Fe14-xAlx+yB25-yC34 may exhibit such a pseudogap, leading to poor metal behavior.

* *

11 Figure 5.6 B MAS-NMR spectrum of Ce33Fe13.1Al1.1B24.8C34. Asterisks denote spinning side bands.

5.3.5 Magnetic Behavior of R33Fe14-xAlx+yB25-yC34 phases

The magnetic properties of R33Fe14-xAlx+yB25-yC34 and R33Fe13-xAlxB14C34 phases are of particular interest, given the presence of two potentially paramagnetic species: the iron clusters and the rare earth cations. The magnetic susceptibility of La33Fe14-xAlx+yB25-yC34 is small and roughly temperature independent (Figure 5.7), indicating Pauli paramagnetic behavior and therefore no magnetic moment on the iron atoms. The slight Curie tail at low temperature is likely due to traces of oxidation or paramagnetic impurities on the surface of the crystal sample. The magnitude of the Pauli paramagnetism (0.06 emu/mol; taking the stoichiometry into account, this corresponds to roughly 1 × 10-3 emu/mol of metal atoms) is consistent with that seen for other metallic compounds [100]. The cerium analog Ce33Fe14-xAlx+yB25-yC34 was studied using susceptibility measurements, XPS measurements to investigate cerium valence,

and Mössbauer studies on an 57-Fe enriched sample to explore possible magnetic transitions of iron sites. The temperature dependence of the susceptibility (Figure 5.7) shows paramagnetic

behavior above 200 K which can be fit to a modified Curie-Weiss law (χ = (C/T-θ) + χp, where C

is the Curie constant, θ is the Weiss temperature, and χp corresponds to the Pauli paramagnetism of the conduction electrons). The resulting parameters, listed in Table 5.4, indicate an effective

magnetic moment per cerium ion of 2.36 µB, which is only slightly lower than the theoretical 3+ value of 2.54 µB for Ce ions [100]. The negative Weiss constant of θ = -8.4K indicates the presence of antiferromagnetic coupling forces, in agreement with the observed antiferromagnetic

ordering at TN = 9 K. This ordering is likely due to the cerium ions; as in the La analog, the iron

atoms in Ce33Fe14-xAlx+yB25-yC34 do not appear to have a magnetic moment.

Table 5.5 Magnetic data for R/Fe/B/C phases 3+ Compound Moment per R θ (K) χp (emu/mol of FU) Observed transition

ion (µB)

La33Fe14B25C34 0.06 Pauli paramagnetic

Ce33Fe14B25C34 2.36 -8.4 0.06 AF; TN = 9K

Ce33Fe13B18C34 Non-Curie-Weiss FM; Tc = 190K

Pr33Fe13B18C34 4.15 -44 0.06 AF; TN = 16K

1.8 12 1.6

1.4 10 1/ χ χ 1.2 χ χ m 8 (mol/emu) 1.0

0.8 6 (emu/mol)

m 0.6 χ χ χ χ 4 0.4 2 0.2 0.0 0 0 50 100 150 200 250 300 Temperature (K)

Figure 5.7 Temperature dependence of magnetic susceptibility for R33Fe14-xAlx+yB25-yC34 phases, with an applied field of 100 Oe. Data for La analog are in black; data for Ce analog (filled circles χm and open circles 1/χm) are in red. 69

The lack of magnetic moment on the iron atoms in R33Fe14-xAlx+yB25-yC34 phases is somewhat surprising considering the magnetic behavior exhibited by iron atoms in the smaller

Fe4 clusters of La21Fe8Sn7C12 [61]. The quenching of the iron magnetism may be due to the

Fe14B clusters of R33Fe14-xAlx+yB25-yC34 being “diluted” by the central boron atom and by the aluminum substitution on the Fe(2) site. Similar dilution effects are observed for R2Fe14B phases and their substituted analogs. In the Nd2Fe14B structure, the iron sites with the largest number of iron nearest neighbors have the largest magnetic moments, and the moments of all the iron atoms are lowered when R2Fe14-xTxB analogs are made with iron diluted by a non-magnetic element T

[76]. The La(AlxFe1-x)13 family of compounds exhibits a similar drop in the iron magnetic moment as the amount of Al substitution increases [101].

The other notable feature in the susceptibility data of Ce33Fe14-xAlx+yB25-yC34 is a broad bump from 75 – 150 K. This is reproducibly seen in the magnetic data for several different samples of this compound. It also corresponds to the kink in the resistivity temperature dependence shown in figure 5.5. Mössbauer spectra were collected at several temperatures spanning this transition to determine if it is caused by the iron sites. The data taken at 78K, 150K, and 294K are shown in figure 5.8; the spectra consist of a broad peak with a shoulder (in agreement with the presence of two iron sites in the structure). No hyperfine splitting or variation in shift is observed between 78 and 294 K. Also, field dependence magnetization data taken at various temperatures in this range (Figure 5.9) show normal linear paramagnetic behavior. Therefore, this transition at around 120 K does not appear to be a magnetic ordering phenomenon and is not associated with the iron sites. The fact that this feature is not seen in the data for the lanthanum analog indicates that it may be due to fluctuating valence of the cerium ions. The Curie-Weiss fit of the susceptibility temperature dependence supports the presence of 3+ Ce at high temperature. This is also supported by room temperature XPS data on Ce33Fe14- xAlx+yB25-yC34 (Figure 5.10) which show the Ce 3d5/2 and 3d3/2 peaks and their satellites in 3+ positions typical for Ce ; the spectrum is very similar to those seen for CePO4 and CeFeAsO [95, 102]. However, as the temperature is lowered below 150 K, thermal contraction of the lattice may become sufficient to promote Ce3+/Ce4+ valence fluctuation. One or more of the three cerium sites may remain trivalent; these remaining paramagnetic ions order antiferromagnetically at 9 K. Similar behavior is seen in the mixed valent phase Ce23Ru7Cd4,

70 where 20 out of the 46 cerium ions in the cell remain trivalent and their magnetic moments order 57 at 3.6 K [70]. The Fe Mössbauer spectra of Ce33Fe14-xAlx+yB25-yC34 (Figure 5.8) show very little change below the antiferromagnetic transition at 9 K; instead of hyperfine splitting, only a slight broadening is observed in the 5.5 K spectrum which indicates weak coupling of the iron with the surrounding Ce3+ ions as they undergo their ordering transition.

Figure 5.8 57Fe Mössbauer spectra at various temperatures for 57-Fe enriched sample of Ce33Fe14-xAlx+yB25-yC34

15 ) 2 K B µ µ µ µ 75 K 10 100 K 5

Magnetization ( 0 -80000 -40000 0 40000 80000

-5 Field (G)

-10

-15

Figure 5.9 Magnetization data for Ce33Fe14-xAlx+yB25-yC34 at several temperatures.

Figure 5.10 X-ray photoelectron spectra for crystals of Ce33Fe13.1Al1.1B24.8C34 after various sputtering times (bottom spectrum unsputtered; other spectra taken at 5 minute increments of sputtering at 2.5 kV). Data shows the Ce 3d5/2 and 3d3/2 peaks and their satellites in positions typical for Ce3+.

5.3.6 Magnetic Behavior of R33Fe13-xAlxB18C34 phases

The magnetic susceptibility data for Ce33Fe13B18C34 is shown in Figure 5.11. The dominant feature is a distinct rise in susceptibility at 180 K and subsequent field-cooled/zero- field-cooled splitting at lower temperatures. Since the room temperature XPS data (Figure 5.12) indicates that this phase contains diamagnetic Ce4+ ions, this rise in susceptibility is likely due to magnetic ordering of the iron clusters. Field dependence data (Figure 5.9) taken above and below the transition at 150 K show an increase in magnetization below this temperature, confirming that the rise in susceptibility is magnetic in origin. The nature of the magnetic ordering is not clear; the magnetization is not saturated, which may indicate that the magnetic coupling and ferri- or ferromagnetic ordering occurs within individual Fe13 clusters, but not between them (each cluster is 12.3Å away from eight neighboring clusters). The data above the ordering temperature cannot be fit to the Curie-Weiss law, so determination of the iron moment is not possible. The fact that the iron atoms in this structure have magnetic moments while the

Fe14 clusters in Ce33Fe14-xAlx+yB25-yC34 do not is likely due to the fact that the Fe13 clusters are not diluted by a central boron atom or small amounts of Al substitution on the iron sites.

2.50

2.00

1.50

1.00 (emu/mol) m χ χ χ χ 0.50

0.00 0 50 100 150 200 250 300 Temperature (K)

Figure 5.11 Temperature dependent magnetic susceptibility for Ce33Fe13B18C34 at 100 Oe applied field.

Figure 5.12 X-ray photoelectron spectra for crystals of Ce33Fe13B18C34 after 20 minutes of 4+ sputtering. The spin-orbit doublet at 900 eV (3d5/2) and 918 eV (3d3/2) is characteristic of Ce (4f0 state). The peaks at 895 eV and 913 eV are shake-down peaks of these transitions.

1.5 )

250 K B µ µ µ µ 200 K 100 K 1

0.5 Magnetization ( 0 -80000 -40000 0 40000 80000 Field (G) -0.5

-1

-1.5

Figure 5.13. Field dependence of magnetization for Ce33Fe13B18C34 at several different temperatures.

The magnetic susceptibility data for the Pr33Fe13-xAlxB18C34 analog, shown in Figure

5.14, can be fit to the Curie-Weiss law above 100 K. This yields a magnetic moment of 4.15 µB 3+ per praseodymium ion, significantly higher than the 3.58 µB expected for Pr [100]. The higher moment indicates that the iron atoms in this phase exhibit magnetic moments, as is seen for

Ce33Fe13B18C34. However, no ferromagnetic ordering is observed that would correspond to the transition at 180 K seen for the cerium compound. This lack of ordering may be due to the small amount of aluminum substitution consistently observed in the EDS and crystallographic data for this analog. This, and the slightly longer Fe-Fe bonds in the iron clusters (2.598Å in Pr33Fe13- xAlxB18C34 vs. 2.579Å in the cerium analog), may hinder ordering of the magnetic moments. The low temperature antiferromagnetic ordering observed at 16 K is likely due to the moments on the Pr3+ ions; antiferromagnetic coupling is supported by the negative Weiss constant of -44 K.

30 4.0

3.5 25

3.0 1/ χ χ χ χ

20 m 2.5 (mol/emu) 15 2.0

(emu/mol) 1.5

m 10 χ χ χ χ 1.0 5 0.5 0 0.0 0 50 100 150 200 250 300 Temperature (K)

Figure 5.14. Temperature dependence of magnetic susceptibility for Pr33Fe13-xAlxB18C34, with an applied field of 100 Oe. Filled black circles χm and open blue circles 1/χm.

5.4 Conclusion The two Ce/Fe/B/C intermetallic compounds grown in Ce/T flux have similar structures, but the cerium and iron atoms in these phases exhibit very different behaviors. The Fe14 clusters in Ce33Fe14-xAlx+yB25-yC34 are non-magnetic due to dilution of Fe-Fe bonding by boron and aluminum; the iron atoms in the Fe13 clusters in Ce33Fe13B18C34 do exhibit magnetic moments and ordering. These two borocarbide-capped iron clusters offer a “missing link” between isolated iron sites in intermetallics and more extended iron building blocks such as iron layers and networks featuring extensive Fe-Fe bonding. Ce33Fe13B18C34 is an ordered array of identical magnetic iron nanoclusters; it is not clear if these relatively isolated Fe13 clusters are interacting. Further studies on the magnetism of this phase are planned, although the synthesis yield must be improved for 57Fe enrichment (for Mössbauer studies) or 11B enrichment to allow neutron diffraction studies. Both Ce/Fe/B/C structures feature three crystallographically unique cerium sites. All of the cerium ions in Ce33Fe14-xAlx+yB25-yC34 are trivalent at high temperature, but some convert to 4+ Ce below 100 K. Ce33Fe13B18C34 appears to feature predominantly tetravalent cerium ions.

Both these behaviors can be explained by the presence of different forms of chemical pressure.

The normal thermal contraction of bonds occurring as Ce33Fe14-xAlx+yB25-yC34 cools is evidently sufficient to trigger conversion of some cerium sites from Ce3+ to Ce4+. The squeezing of cerium 3+ ions into the R33Fe13B18C34 structure (which is more stable for smaller Pr cations) stabilizes the tetravalent state. Experiments to substitute smaller R3+ cations into these structures to observe the effects are underway.

Chapter Six

Intermetallics as a Catalyst for Carbon Nanotube Growth

6.1 Introduction to Carbon Nanotubes Carbon nanotubes (CNT) are described as a single sheet of graphene rolled into a tube. Depending on the growth conditions and catalyst, they can be single-walled (SWCNT) or multiwalled (MWCNT). A SWCNT can be described by various orientations depending on how the graphene layer wraps. These three shapes are; armchair (n,n), zigzag (n,0), and chiral (n,m) which have important consequences in their physical properties. MWCNT are concentric single walled carbon nanotubes. In figure 6.1 images of SWCNT [103] and MWCNT [104] are shown. SWCNT can grown in bundles which are long and fiber-like. It is more likely these tubes are uniform in shape. MWCNT on the other hand are thick in appearance and can be bent and disordered and do not possess a uniform shape. There are many factors that play a role in the growth of high quality CNTs: synthetic method, catalyst, substrate, and flow rate of the carbon source. The main ways to synthesize CNTs are arc discharge, laser ablation, and chemical vapor deposition (CVD). In arc discharge methods, CNTs are observed in the carbon soot at the graphite electrodes and both SWCNTs and MWCNTs are observed in lengths up to 50µm. Laser ablation methods require the use of a pulsed laser to vaporize the graphite target and an inert carrier gas blows into a high temperature reactor towards a cooled surface on the other end, and the tubes are collected. The most effective method for producing high quality CNTs is CVD synthesis. CVD synthesis involves the reaction of a gaseous hydrocarbon with a catalytic compound. The most common catalysts are 1-2 nm sized transition metal (commonly Fe, Ni, Mo, Co) nanoparticles. These nanoparticles can be patterned on the surface of a substrate to form ordered arrays of carbon nanotubes for applications such as hydrogen storage, electrochemical devices, and composites [105]. The carbon source (CH4, C2H4, C2H2, C3H8, etc.) is flowed over the material and heated at high temperatures (600°C -900°C) for some length of time. If there is a buildup of amorphous carbon, which poisons the catalyst, an oxidizing agent such as H2 is flowed as a mixture with the carbon source. Catalysis of CNT can occur in two different ways and can be classified as base-grown or tip-grown. Base growth is observed when a strong substrate-catalyst interaction is present and 77 the catalyst will stay in the substrate material, absorb the carbon gas, then precipitate the CNT. Tip-grown CNT are caused by weak substrate-catalyst interaction when the catalyst absorbing the gas and the CNT precipitating underneath the catalyst pushes the catalyst out of the substrate. SEM can be used to make the determination of the different types of growth because the catalyst in tip-growth will be visible as beufgr tips. (figure 6.2) [106,107].

a) b)

Figure 6.1: a) TEM image of a bundle of SWCNT b) TEM image of MWCNT [103,104].

Figure 6.2 Generally accepted method for CNT growth where a) is tip growth and b) is base growth [106].

Raman spectroscopy is extremely useful in characterizing the nature of CNT grown using the CVD process. From the Raman experiments, observations of 4 bands are crucial for the determination of the size and shape of the CNT. The D band at ~1350 cm-1 is the defect band. This gives information about the defects present in the tube. For high quality CNT this band should be smaller than the G band (found between ~1550-1605cm-1) which describes the

78 graphitic nature of the material. The G’ band, ~2700cm-1 describes the overtone of the D band and is highly dispersive [108] The radial breathing mode (RBM) is found in the 100 – 300 cm-1 range and is only observed for SWCNT. This is an in-phase displacement band that provides information about the expansion and contraction of the tube and thus its size.

6.2 New catalysts for CNT growth Many different metals have been explored as catalysts for CVD growth of carbon nanotubes, but the majority of work in this field has focused on iron, cobalt, or nickel nanoparticles [109]. These were originally thought to be the only catalysts capable of yielding single walled CNTs, although recent work using Au and Pd nanoparticles has proven otherwise

[110, 111]. Binary alloys such as LaNi5 and Fe/Zr alloys have also been explored, although the studies concluded that most of the catalytic activity occurred at islands of Ni or Fe respectively [112, 113]. There is currently not a clear understanding of how catalyst composition controls the characteristics (diameter, chirality, ratio of metallic/semiconducting nanotubes) of the CNT products. A comparison between the catalytic abilities of intermetallic phases containing iron building blocks of various sizes may shed light on the growth mechanism. Expanding on the work of iron nanoparticle catalysis, we investigated whether complex iron-containing intermetallics in bulk crystalline form can also catalyze CNT growth. Phases such as Y5Mg5Fe4Al12Si6, Ce21Fe8Si7C12, Ce33Fe14B25C34, and La6Fe10Al3Si can be grown from metal fluxes as large crystals [114, 61, 115, 46]. The structures of these compounds contain iron building blocks of varying sizes, as shown in Figure 6.3. Y5Mg5Fe4Al12Si6 does not feature any Fe-Fe bonding; instead, the iron atoms in the structure are surrounded by Al and Si atoms.

Ce21Fe8Si7C12 contains Fe4 tetrahedra capped on each side by carbon atoms; these Fe4C6 units are surrounded by a Ce/Si network. Ce33Fe14B25C34 can be viewed as a body centered cubic array of 3+ Fe14B clusters capped with borocarbide chains and surrounded by Ce cations. La6Fe10Al3Si has 2-D slabs of linked Fe icosahedra separated by La-rich layers. We have found that the ability of these phases to catalyze the conversion of methane to CNT appears to be directly linked to the dimensionality and extent of Fe-Fe bonding of the iron building block.

Figure 6.3: Structures and chemical formulas of each of the crystals studied. Isolated to layers of Fe atoms were used to see which would catalyze the growth of CNT more effectively.

6.3 Materials and Methods 6.3.1 Synthesis In this work, methane was reacted with a variety of crystalline intermetallic catalysts in a CVD process and the resulting products on the surface of the crystals were analyzed using SEM,

TEM, XPS, and Raman spectroscopy. The synthesis of Ce33Fe14B25C34, La6Fe10Al3Si, and

Y5Mg5Fe4Al12Si6 were carried out as described previously[114,115,46]; crystal growth of

Ce21Fe8Si7C12 from Ce/Co flux followed the method reported for its lanthanum analog

La21Fe8Sn7C12 (see Chapter 4) [61]. Chemical vapor deposition experiments were carried out using a tube furnace and a 2.5 cm diameter quartz tube. Source gases were Airgas 100% methane (UN1971) and Airgas UHP nitrogen (UN1066). The intermetallic catalysts are slightly air-sensitive, so care was taken to avoid exposure to trace oxygen or water, particularly at high temperatures. Large (1 – 2 mm diameter) crystal samples of the three phases were placed into individual alumina boats positioned in the center of the quartz tube. UHP nitrogen was directed through the tube as the temperature was ramped from room temperature to 690°C in 12°C/min. Once the process temperature was reached, methane gas was directed over the catalyst crystals at a flow rate of 1 mL/min, for up to 120 minutes. After the set reaction time, nitrogen gas was reintroduced and the furnace cooled to room temperature.

6.3.2 Scanning Electron Microscopy and Transmission Electron Microscopy The resulting growth on the surface of the catalyst crystals was imaged with SEM and TEM. For Scanning Electron Microscopy, a FEI Nova 400 Nano SEM was used. The samples were prepared by affixing the crystals to a sample stub with carbon tape; an accelerating voltage of 5 kV was used for imaging. For TEM imaging, the sample was crushed and a colloidal methanol solution containing the crushed sample was dropped onto the surface of a C/Cu grid. TEM images were collected on a Philips CM120 instrument.

6.3.3 X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy was performed using a Physical Electronics PHI 5100 series XPS with a non-monochromated dual anode (Al & Mg) source having a single channel hemispherical energy analyzer. A Mg Kα source was used. Sputtering of the sample was carried out to remove any surface oxidation and physisorbed species. XPS spectra were taken after every 5 minutes of sputtering until no further changes in the spectra were observed. A total of 10 min of sputtering was sufficient to eliminate oxygen from the sample surface.

6.3.4 Raman Spectroscopy Raman spectra were obtained using a micro-Raman spectrograph, JY Horiba LabRam HR800, excited by a TUI Optics DL 100 grating-stabilized diode laser emitting 80 mW of power at 633 nm. The power at the sample was 6 mW. The spectrograph uses a holographic notch filter to couple the laser beam into the microscope (Olympus BX30) by total reflection. The beam was focused on the sample through a 50x microscope objective (Olympus N. A. 0.10). Backscattered radiation was collected by the objective, and laser radiation was filtered out by the notch filter with Raman scattering coupled into the spectrograph through a confocal hole. A 76 mm square 600 line/mm grating dispersed the Raman scattering onto a 1024 x 256 element open electrode CCD detector (Wright CCD30-11-0-275) having 26 µm square pixels thermoelectrically cooled to -70 °C. The detector has a quantum efficiency of 45-22% in the range of 785-900 nm.

‘

6.4 Results and Disucssion Numerous trials were carried out to investigate the effects of reaction time, temperature, and catalyst on the growth of CNT. Ce33Fe14B25C34 appears to be the most reactive catalyst studied in this work. The SEM and TEM images obtained for growth on this phase at a reaction temperature of 690°C are shown in Figure 6.4 (reactions at lower temperature of 590°C did not produce any product). For short reaction times at 690°C (25 minutes), there is no clear evidence of CNT formation. The darker regions on the SEM image suggest preliminary graphitic growth on the surface of the crystal (supported by Raman data, vide infra). After 30 and 45 min, CNT growth is evident coating of the sides of the crystal. After 1 hour of reaction with methane, the TEM image clearly shows long SWCNT growing from the surface of the crystal. The growth is not uniformly distributed on the surface of the crystal, and there does not appear to be a preferred facet or orientation for growth.

Figure 6.4 SEM (left column) and TEM (right column) images of CNT growth on the surface of Ce33Fe14B25C34 crystals after reaction with methane at 690°C. a) 25 min, b) 30 min, c) 45 min, and d) 60 min of reaction time.

Raman spectra can be used to determine whether a carbon nanotube sample is single-wall or multi-wall, and to analyze its diameter [116]. The ratio of the intensities of the G-peak (graphene mode, 1565-1595 cm-1) and the D-peak (disorder-induced band at 1300 cm-1; intensity increases with number of structural defects which are commonly seen for MWCNT) is a good measure of the quality of the sample; the higher the ratio, the higher the yield of SWCNT. Splitting of the G-band into G+ and G- modes is also indicative of SWCNT [117]. The Raman spectra obtained for CVD reaction products grown on the surface of Ce33Fe14B25C34 after varying reaction times are shown in Figure 6.5. The ratio of G-band to D-band intensity increases as the growth time increases, with G-band splitting appearing after an hour. This indicates that the initial growth on the surface of the crystal is highly disordered, including graphitic regions and possibly MWCNTs. After 45 minutes, SWCNT growth becomes apparent in both the TEM images and Raman spectra.

Figure 6.5 Raman spectra of CNT growth on Ce33Fe14B25C34 after varying amounts of reaction time with methane at 690°C, and associated TEM images.

The radial breathing modes (RBM) for carbon nanotubes typically occur in the Raman spectrum in the 150 – 350 cm-1 range. These modes are proportional to the diameter of the

-1 7 CNTs according to the equation dt(nm) = 233 cm / ωRBM. For the nanotubes grown on the -1 -1 Ce33Fe14B25C34 catalyst, a RBM peak appears at 141 cm after 25 minutes, 169 cm after 30 minutes, 257 cm-1 after 45 minutes, 163 cm-1 after 60 minutes and 229 cm-1 after 120 minutes (see Figure 6.6). These correspond to diameters of 1.65 nm, 1.38 nm, 0.91 nm, 1.43 nm, and 1.02 nm. There does not appear to be a relationship between time of growth and nanotube diameter. This is likely due to a range of diameters being present and the fact that the SWCNT’s are bundled and not isolated from each other. A full characterization of the diameter distributions present in the products using varying laser wavelengths is needed to verify this.

Figure 6.6. The radial breathing mode region of Raman spectra of CNT growth on Ce33Fe14B25C34 after varying amounts of reaction time.

XPS data were obtained for the CNT products grown on Ce33Fe14B25C34 after 60 minutes of reaction with methane at 690°C. Physisorbed oxygen was removed by sputtering. The peak associated with the C-C bond was observed at 284.5eV and the πà π* transition was observed at 291.5eV (Figure 6.7). There are no additional peaks between these binding energies that would suggest that the sample contains defective CNTs [118].

Figure 6.7. XPS spectra of surface growth on Ce33Fe14B25C34 after 60 min of reaction with methane at 690°C. The peaks at 284.5eV and 291.5eV after 10 min of sputtering to remove surface oxides indicate well-formed CNT.

Methane did not appear to react with bulk iron or La6Fe10Al3Si at 690°C. Reactions did occur with Y5Mg5Fe4Al12Si6 and Ce21Fe8Si7C12 samples, but did not produce SWCNT products.

Use of Y5Mg5Fe4Al12Si6 led to formation of amorphous carbon on the surface of the crystal indicated in SEM and TEM images and Raman spectra (Figures 6.8 and 6.9). Reactions using

Ce21Fe8Si7C12 as the catalyst lead to formation of MWCNT (Figures 6.8 and 6.9). No growth was seen for either of these compounds at a lower temperature of 590ºC (Figure 6.10).

Figure 6.8. SEM (left) and TEM (right) images after reaction with methane for 60 min at 690°C. a) Y5Mg5Fe4Al12Si6, showing no nanotube growth; b) Ce21Fe8Si7C12, showing growth of MWCNT.

Figure 6.9 Raman spectra of Y5Mg5Fe4Al12Si6 (red) and Ce21Fe8Si7C12 (blue) after 60 minutes of reaction with methane at 690 °C. The large D-band and lack of RBM indicates that growth on Ce21Fe8Si7C12 is MWCNT; lack of distinct peaks in the spectrum for Y5Mg5Fe4Al12Si6 indicates no CNT growth.

Figure 6.10 SEM (left) and TEM (right) images of a) Y5Mg5Fe4Al12Si6 and b) Ce21Fe8Si7C12, after 60 min reaction with methane at 590°C. No reaction is observed at this temperature.

6.5 Conclusion In summary, conversion of methane to carbon nanotubes can be catalyzed using bulk intermetallics which feature a certain amount of iron connectivity in their structures.

Compounds with no Fe-Fe bonding (such as Y5Mg5Fe4Al12Si6) or with iron building blocks of two dimensions or higher (the 2-D slabs in La6Fe10Al3Si, or 3-D connectivity of bulk iron), do not catalyze formation of CNT. However, phases featuring 0-D clusters of iron (such as the Fe4 clusters in Ce21Fe8Si7C12 or the Fe14B clusters in Ce33Fe14B25C34) are active in producing nanotubes. An additional benefit is their degradation in water, which should make them easier to remove from the CNT products than traditional iron nanoparticle catalysts. Intermetallic phases featuring 0-D iron clusters in their structures are fairly rare. Aside from the compounds studied here, there are Fe6 clusters in Er15Fe8C25 [119], and Fe13 clusters in Ce33Fe13B18C34 [115]; these compounds may also promote conversion of hydrocarbons to CNT. Other intermetallic phases which may have catalytic activity include Ti7Fe4Ru18B8 (which features 1-D chains and ladders of iron) [120] and possibly iron-containing analogs of the ThCr2Si2 structure type such as

BaFe2As2 (which contain 2-D square nets of iron atoms) [121]. More work is needed to explore the nature of the interaction between methane and bulk intermetallics. The diameters of the CNT products do not appear to scale directly with the size of the iron nanoclusters in the structure, which may indicate that the surface of the catalyst undergoes significant rearrangement or degradation during the growth process. Further studies on other single crystals of phases containing different clusters of Fe, Co, and Mn will shed light on this.

Chapter Seven

Rare Earth Carbides and Borocarbides Grown from Ce/Co, Ce/Fe, and Ce/Ni Eutectic Flux

7.1 Introduction While exploratory reactions in common metal fluxes such as Sn, In, and Al have yielded many new phases, use of metal fluxes with higher melting points is more difficult. For growth of magnetic phases rich in rare earths and transition metals, a reactive flux containing such elements is desirable. Most combinations of early rare earths (La, Ce, Pr, Nd) with late transition metals (Co, Ni) have a specific ratio that forms a low-melting eutectic that can be used as a flux. The lowered melting point allows for lower reaction temperatures and facilitates crystal growth of metastable phases. We have extensively explored reactions of various elements using the

La/Ni eutectic (67:33, 517°C) [6] as a flux, producing phases such as La21Fe8Sn7C12,

La14Sn(MnC6)3, and LaRu2Al2B [61, 67, 66]. The presence of rare earth and transition metals in the reactions are desired to yield interesting magnetic properties. Main group elements are added to produce structural diversity in the products. Carbon is of particular interest in this regard; it is soluble in RE/T fluxes and exhibits a wide variety of bonding motifs in the resulting intermetallics. A recent review of ternary RE/T/C phases classifies them into two groups: carbometallates (featuring isolated transition metals coordinated by several carbon atoms, resulting in anionic TxCy units, chains, or networks) and metal-rich carbides (featuring extensive T-T bonding and small amounts of interstitial carbide atoms) [8]. A common repeating unit in carbometallates is a trigonal planar

TC3 unit. This has been observed in La14Sn(MnC6)3 [67], La3.67[Fe(C2)3] [122], and RE15Fe8C25

[119]. Other similar units such as transition metals linked by C2 bridges were observed in

Sc3TC4 (T = Fe, Co, Ni, Ru, Rh, Os, Ir) [123]. Magnetic behavior of these materials has been attributed to the interaction of the RE cation with its surrounding environment. Anions of [TyCz]-n in these structures do not play a direct role in the nature of the magnetic behavior [8].

In this work, the phases Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 and Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 with a new zeolite-like structure were synthesized using Ce/Co, Ce/Ni, and Pr/Ni eutectic fluxes. This structure features corner sharing FeC3 trigonal planar units that form rings connected to one another by Fe-Fe bonding. Aluminum or S coordinated by 9 R atoms center these 9Å rings.

Other structures such as Ce7Fe2C9 and Ce4FeGa1-xAlxC4 (x= 0.15,1.0), were synthesized serendipitously from exploratory reactions in attempts to optimize and synthesize analogs.

These structures contain Fe2C8 and FeC4 units respectively.

7.2 Materials and Methods 7.2.1 Synthesis Starting materials were stored and handled in an argon-filled dry-box. Powders of the following elements were used in synthesis: carbon black (95-97% Strem Chemicals), boron (99% Strem Chemicals), iron (99% Alfa Aesar), and aluminum (99% Alfa Aesar). Ce/Co, Ce/Ni, and Ce/Fe eutectics were synthesized from Ce ingot with Co, Ni, or Fe slug (99% Alfa Aesar) and were combined in a 76:24, 78:22, or 81:19 atomic ratio respectively [6]. Pr/Ni eutectic was synthesized by using Pr ingot (99% Alfa Aesar) and Ni slug (99% Alfa Aesar) and combining them in an 81:18 atomic ratio. All eutectics were synthesized by arc melting their respective elements on a water cooled copper hearth into a button that was turned over several times and re-melted to ensure homogeneity of the flux material. The eutectic was broken up into approximately 1mm3 pieces. Reactants of C, B, Fe, and Al (1,1,1,1 mmol respectively) were sandwiched between layers of eutectic flux with more on the top than the bottom. This allows reactants that have lower densities to mix with the eutectic as it melts. Once the crucible was loaded with the desired reactants and placed into a quartz tube, a piece of silica wool was used as a filter for centrifugation. The silica tube containing the steel crucible and reactants were fused under a vacuum of 10-2 Torr. The ampoule was heated to 950°C in 3 hours, held at this temperature for 12 hours, and then cooled to 850°C in 10 hours. The reaction mixtures were subsequently annealed for 48 hours at 850°C and then cooled to 600°C in 84 hours. At 600°C the ampoule was removed from the furnace, quickly inverted, and placed into a centrifuge to decant the molten flux. Ce/Fe eutectic reactions were centrifuged at 700°C to accommodate the higher melting point of the eutectic. For Ce/Ni and Pr/Ni eutectic reactions the heating profile was modified, dropping the maximum temperature of the reaction to 850°C, cooling to 700°C in 10 hours, holding for 12, and slowly cooling to 600°C in 84hours. Lowering the maximum temperature for Pr and Ce/Ni eutectic reactions helped promote the growth of

Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 as well as the Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 and Ce5(Fe/Ni)3C4structures.

Stoichiometric synthesis of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, was attempted by combining elemental forms of Ce, Fe, B, and C in a 30,12,6,13,4 mmol ratio in an alumina crucible. These reactions were prepared in an argon-filled dry-box and the crucible was placed in a fused silica tube and sealed under a vacuum of 10-2 Torr. The ampoule was then heated to 950°C in 3 hours, held at this temperature for 24 hours, and then cooled to 25°C in 3 hours. Products were stored in an argon-filled dry-box to prevent oxidation of the powders.

7.2.2 Elemental Analysis Elemental analysis was performed on all samples using a JEOL 5900 scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDXS) capabilities. Flux-grown crystals from each reaction were affixed to an aluminum SEM puck using carbon tape, and positioned so that flat faces were perpendicular to the electron beam. Samples were analyzed using a 30kV accelerating voltage and an accumulation time of 60s. The data indicated the presence of Ce, Fe, and Al in approximately a 70:20:2 ratio; no cobalt was detected. The carbon and boron content was not able to be determined due to the limitation of the EDXS in detecting the characteristic X-rays of light elements. Analysis of light elements was performed on samples previously screened by EDS, using a Physical Electronics PHI 5100 series XPS with a non-monochromated dual anode (Al & Mg) source having a single channel hemispherical energy analyzer. Large single crystals of this phase were affixed to a carbon coated sample puck using carbon tape. The Mg and Al X-ray sources were used to study all elements present in the sample. To eliminate surface species (oxides, residual flux coating), samples were sputtered for a total of 15 minutes by Ar+ ions as the sample stage was rotated. Spectra were taken after every five minutes of sputtering to monitor for the formation of new species. Once no new species were formed no additional sputtering was necessary.

7.2.3 X-ray Diffraction Selected single crystals were mounted on cryo loops using paratone oil. The X-ray intensity data were collected at 150K on a Bruker SMART APEX2 CCD diffractometer equipped with a Mo-target X-ray tube (λ = 0.71073). The data sets were recorded as ω scans at 0.3° stepwidth and integrated with the Bruker SAINT software [43]. Data were corrected for

absorption effects using the semi-empirical method (SADABS [43]). The structure was refined in orthorhombic space group Pmmm (space group #47) by full matrix least squares procedures on |F2| using the SHELX-97 software package [44]. To test for the incorporation of Al, all site occupancies were allowed to vary in the final refinement cycles. Unit cell parameters and crystallographic data collection information are found in table 7.1-7.2 and atomic coordinates and displacement parameters are found in table 7.3-7.8 Selected bond lengths are found in table 7.9-7.10.

Table 7.1 Crystallographic data collection for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, Pr31.6(5)Fe10.4(4)S6.6(4)B12C5, and Ce5(Fe/Ni)3C4 Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 Ce5(Fe/Ni)3C4 Formula Weight 10938.64 10889.24 916.19 (g/mol) Crystal System Orthorhombic Space group Pmmm P21/m a, Å 11.3888(8) 11.378(1) 5.272(1) b 15.580(1) 15.635(2) 20.417(4) c 15.581(1) 15.637(2) 13.829(3) Z 2 6 Volume 2764.78 2782.07 1488.69 Density, calc 6.570 6.499 6.132 (g/cm3) Radiation Mo Kα Temperature(K) 150K Index Ranges -15≤h≤15 -14≤h≤14 -6≤h≤6 -20≤k≤20 -20≤k≤19 -25≤k≤26 -20≤l≤20 -20≤l≤20 -18≤l≤17 Reflections 30655 31869 16825 Collected Unique 3716/209 3748/208 3837/212 Data/Parameters

µ (mm-1) 27.99 29.96 26.607 R1/wR2 0.0394/0.0965 0.0305/0.0614 0.0476/0.1198 R1/wR2 (all 0.0251/0.1053 0.0421/0.0696 0.0499/0.1222 data) Residual 7.03/-9.79 7.968/-4.146 9.389/-8.170 peak/hole e-Å-3

Table 7.2 Crystallographic data collection for Ce4FeAlC4, Ce7Fe4C9, and Ce4FeGa0.85Al0.15C4

Ce4FeAlC4 Ce7Fe4C9 Ce4FeGa0.85Al0.15C4 Formula Weight 172.84 1200.63 735.48 (g/mol) Crystal System Orthorhombic Monoclinic Monoclinic Space Group Pcca P21/c C2/m a, Å 5.77560(7) 8.7632(5) 5.7843(5) b 8.297(1) 7.4926(5) 15.359(1) c 15.476(1) 10.5016(5) 8.2809(7) β 121.457(4) 91.369(1) Z 4 2 4 Volume 739.22 588.19 735.48 Density, calc 1.553 6.779 1.968 (g/cm3) Radiation Mo Kα Temperature 150K Index Ranges -15≤h≤15 -11≤h≤12 -7≤h≤7 -20≤k≤20 -9≤k≤9 -20≤k≤20 -20≤l≤20 -13≤l≤13 -10≤l≤10 Reflections 887 1373 894 Collected Unique 887/49 1373/86 894/52 Data/Parameters µ (mm-1) 6.52 28.83 7.30 R1/wR2 0.0180/0.0436 0.0189/0.0432 0.0169/0.0341 R1/wR2 (all 0.0243/0.0458 0.0209/0.0434 0.0179/0.0344 data) Residual 1.369/-0.765 1.922/-1.664 1.079/-0.747 peak/hole e-Å-3

Table 7.3: Atomic positions for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 (Z =2) Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 Atom Wyckoff Occupancy x y z Ueq site Ce1 8a 0.16388(6) 0.14017(5) 0.37876(5) 0.0111(1) Ce2 8a 0.16389(6) 0.37871(5) 0.14013(5) 0.0112(1) Ce3 8a 0.23598(7) 0.14029(5) 0.14025(5) 0.0130(1) Ce4 4z 0.2170(1) 0.34484(8) ½ 0.0170(2) Ce5 4x 0.2169(1) ½ 0.34481(7) 0.0169(2) Ce6 4v ½ 0.14476(7) 0.32998(6) 0.0099(2) Ce7 4v ½ 0.33008(7) 0.14474(6) 0.0102(2) Ce8 4v ½ 0.37503(7) 0.37494(6) 0.0097(2) Ce9 4u 0.987 0 0.34155(8) 0.34148(8) 0.0171(4) Ce10 2s ½ 0 0.1434(1) 0.0227(3) Ce11 2q 0 0 0.22838(9) 0.0105(3) Ce12 2o ½ 0.1433(1) 0 0.0225(3) Ce13 2m 0 0.2284(1) 0 0.0108(3) Ce14 2k 0.3505(1) ½ 0 0.0108(3) Ce15 2j 0.3504(1) 0 ½ 0.0108(2) Ce16 1e 0 ½ 0 0.066(1) Ce17 1c 0.588 0 0 ½ 0.026(1) Ce18/Fe18 1a 0 0 0 0.0073(8) Fe1 8a 0.2706(1) 0.3011(1) 0.3010(1) 0.0117(3) Fe2 4z 0.3857(2) 0.2034(1) ½ 0.0070(8) Fe3 4x 0.961 0.3855(2) 11/2 0.2036(1) 0.0074(8) Fe4 4u 0.951 0 0.2064(1) 0.2067(1) 0.0096(5) Fe5 2i 0.3054(8) 0 0 0.071(2) Fe6 1g 0 ½ ½ 0.014(1) Al1 4y 0.2989(4) 0.2847(3) 0 0.007(1) Al2 4n 0.2990(4) 0 0.2849(3) 0.007(1) Al3 2r 0.884 0 ½ 0.2361(8) 0.023(3) Al4 2n 0 0.2360(9) ½ 0.033(2) Al5/Fe5A 2l 0.673/0.327 0.3772(8) ½ ½ 0.030(2) C1 4u 0 0.116(1) 0.118(1) 0.013(3) C2 2t ½ ½ 0.111(1) 0.009(4) C3 2p ½ 0.110(1) ½ 0.009(4) B1 8a 0.140(1) 0.2499(7) 0.2498(7) 0.001(2) B2 8a 0.335(1) 0.3969(7) 0.2487(7) 0.001(2) B3 8a 0.336(1) 0.2489(8) 0.3972(7) 0.003(2) B4 2i 0.149(2) 0 0 0.002(4)

Table 7.4: Atomic positions for Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 (Z=2) Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 Atom Wyckoff Occupancy x y z Ueq site Pr1 8a 0.16660(5) 0.37707(3) 0.35813(4) 0.0095(1) Pr2 8a 0.16654(5) 0.14191(4) 0.12286(3) 0.0094(1) Pr3 8a 0.23530(5) 0.14045(3) 0.35956(3) 0.0100(1) Pr4 4y 0.22117(7) 0.34293(5) 0 0.0127(1) Pr5 4x 0.22109(7) ½ 0.15707(5) 0.0127(1) Pr6 4v ½ 0.37535(5) 0.12566(5) 0.0080(1) Pr7 4v ½ 0.14520(5) 0.16867(5) 0.0078(1) Pr8 4v ½ 0.33129(5) 0.35478(5) 0.0078(1) Pr9 4u 0 0.33968(5) 0.16036(5) 0.0112(1) Pr10 2s ½ 0 0.35270(8) 0.0145(2) Pr11 2s 0.703(8) 0 ½ 0 0.0216(9) Pr12 2q 0 0 0.27259(7) 0.0083(2) Pr13 2p ½ 0.14711(8) ½ 0.0144(2) Pr14 2o 0 0.22738(7) ½ 0.0086(2) Pr15 2l 0.35192(9) ½ ½ 0.0081(2) Pr16 2i 0.35185(9) 0 0 0.0078(2) Pr17 1g 0.819(8) 0 ½ ½ 0.0176(7) Pr18 1c 0.966(7) 0 0 ½ 0.0098(5) Pr19 1a 0.804(8) 0 0 0 0.0173(7) Fe3 8a 0.2744(1) 0.29855(9) 0.20148(9) 0.0085(2) Fe4 4y 0.3864(1) 0.2039(1) 0 0.0078(4) Fe5 4x 0.3862(1) ½ 0.2960(1) 0.0080(4) Fe2 4u 0 0.2066(1) 0.2933(1) 0.0082(4) Fe1 2j 0.3151(5) 0 ½ 0.008(1) S3 4z 0.66(2) 0.2979(3) 0.2843(2) ½ 0.015(1) S2 4w 0.97(1) 0.2978(3) 0 0.2157(2) 0.0156(7) S4 2r 0 ½ 0.2682(9) 0.039(4) S5 2m 0 0.229(1) 0 0.090(4) S1 2k 0.3805(8) ½ 0 0.036(1) C4 4u 0 0.1184(9) 0.3811(8) 0.012(2) C3 2t ½ ½ 0.389(1) 0.012(4) C1 2o ½ 0.111(1) 0 0.009(3) C2 2j 0.145(2) 0 ½ 0.034(6) B1 8a 0.1418(8) 0.2471(6) 0.2520(6) 0.001(1) B2 8a 0.3394(8) 0.2483(6) 0.1038(5) 0.002(1) B3 8a 0.3394(8) 0.3955(6) 0.2521(6) 0.001(1)

Table 7.5: Atomic positions for Ce4FeGa0.85Al0.15C4 (Z = 4). Ce4FeGa0.85Al0.15C4 Atom Wyckoff Occupancy x y z Ueq site Ce1 8j 0.25313(4) 0.16038(2) 0.35868(3) 0.00618(9) Ce2 4i -0.24166(6) 0 0.26863(4) 0.0066(1) Ce3 4g 0 0.31596(2) 0 0.0062(1) Ga1 4i 0.904 0.2568(1) 0 0.1047(1) 0.0073(2) Fe1 4g 0 0.12572(2) 0 0.0078(2) C1 8j 0.1370(7) 0.1647(3) -0.1892(5) 0.0076(8) C2 8j 0.2424(8) 0.3305(3) 0.3277(5) 0.0105(9)

Table 7.6: Atomic positions for Ce4FeAlC4 (Z=4) Ce4FeAlC4 Atom Wyckoff site x y z Ueq Ce1 8f 0.00309(5) 0.35768(3) 0.08771(2) 0.0065(1) Ce2 4d ¼ 0 0.06566(3) 0.0072(1) Ce3 4c 0 0.72554(5) ¼ 0.0072(1) Fe1 4d ¼ 0 0.37494(6) 0.0068(2) Al1 4c 0 0.0998(2) ¼ 0.0079(4) C1 8f 0.3895(9) 0.1891(6) 0.4142(3) 0.008(1) C2 8f 0.4927(9) 0.3302(6) 0.0798(3) 0.009(1)

Table 7.7: Atomic positions for Ce7Fe2C9 (Z=2) Ce7Fe2C9 Atom Wyckoff Site x y z Ueq Ce1 2b ½ 0 0 0.0044(1) Ce2 4e 0.26049(4) 0.77152(4) 0.13412(3) 0.00556(9) Ce3 4e 0.27056(4) 0.31381(4) 0.09872(3) 0.00563(9) Ce4 4e 0.00263(4) 0.50697(4) 0.75175(4) 0.00734(9) Fe1 4e 0.4211(1) 0.0852(1) 0.38017(9) 0.0093(1) C1 4e 0.7799(7) 0.2175(7) 0.1339(6) 0.007(1) C2 4e 0.6491(7) 0.3523(7) 0.0696(1) 0.004(1) C3 4e 0.2462(7) 0.2566(7) 0.3343(6) 0.007(1) C4 4e 0.4690(7) 0.0488(7) 0.2276(6) 0.007(1) C5 2c 0 0 ½ 0.017(1)

Table 7.8: Atomic positions for Ce5(Fe/Ni)3C4 Ce5(Fe/Ni)3C4 Atom Wyckoff x y z Ueq site Fe1 4c 0.1912(4) 0.2731(1) 0.4691(1) -0.0053(4) Fe2 4c 0.1839(5) 0.2247(1) 0.7602(1) -0.0046(4) C1 2b ½ 0.049(2) -0.195(3) 0.02(1) Fe3 2b ½ 0.0179(2) 0.3926(3) 0.009(1) Ce1 2b ½ 0.05525(9) 0.1962(1) 0.0042(3) Ce2 2b ½ 0.11118(9) 0.55643(1) 0.0067(3) Ce3 2b ½ 0.12758(9) 0.9535(1) 0.0047(3) C2 2b ½ 0.172(1) 0.3398(1) -0.005(3) Ce4 2b ½ 0.23211(9) 0.2205(1) 0.0062(3) Fe4 2b ½ 0.2350(1) 0.7551(2) 0.075(1) Fe5 2b ½ 0.2759(1) 0.4753(2) 0.0006(6) C3 2b ½ 0.3530(9) 0.323(1) -0.008(3) Fe6 2b ½ 0.3607(2) 0.0963(4) 0.013(1) Ce5 2b ½ 0.4413(1) 0.4628(1) 0.0146(4) C4 2b ½ 0.448(1) 0.038(3) 0.013(7) Ce6 2b ½ 0.4886(1) 0.2066(1) 0.0080(3) Ce7 2b ½ 0.6185(1) 0.3779(2) 0.0182(5) C5 2b ½ 0.6672(8) 0.141(1) -0.016(3) Ce8 2a 0 0.01230(9) 0.0190(1) 0.0054(3) C6 2a 0 0.054(1) 0.186(2) 0.006(6) Ce9 2a 0 0.06176(9) 0.7557(1) 0.0041(3) C7 2a 0 0.1107(9) 0.564(1) -0.002(4) Ce10 2a 0 0.11858(9) 0.35552(1) 0.0061(3) Fe7 2a 0 0.1388(2) 0.1325(3) 0.009(1) C8 2a 0 0.1417(9) -0.000(1) -0.016(3) C9 2a 0 0.2075(8) 0.219(1) -0.012(3) Ce11 2a 0 0.26684(8) 0.0070(1) 0.0051(3) Ce12 2a 0 0.3721(1) 0.2731(1) 0.0056(3) Ce13 2a 0 0.44739(9) 0.0406(1) 0.0056(3) C10 2a 0 0.450(1) 0.417(2) 0.007(6) C11 2a 0 0.4931(8) 0.199(1) -0.014(3) Fe8 2a 0 0.5205(2) 0.3296(3) 0.0057(9) C12 2a 0 0.6089(8) 0.347(1) -0.017(2) Ce14 2a 0 0.6189(1) 0.1598(1) 0.0282(6)

7.9 Selected bond lengths for Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, Pr31.6(5)Fe10.4(4)S6.6(4)B12C5and Ce5(Fe/Ni)3C4 Bond Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 Ce5(Fe/Ni)3C4 Length Å Length Å Length Å R-R 3.4382(2)-3.9034(8) 3.4734(9)-4.1005(7) 3.541(2)-4.005(2) R-Fe 3.146(7)-3.457(1) 3.065(1)-3.282(1) 3.042(5)-3.22(3) R-M 2.964(7)-3.517(5) 3.057(8)-3.223(2) N/A R-C 2.698(4)-2.733(3) 2.41(1)-2.62(1) 2.47(3)-2.716(8) R-B 2.57(1)-2.65(1) 2.575(9)-2.70(1) N/A Fe-Fe 2.602(5) 2.584(4) N/A Fe-C 1.94(1)-1.96(1) 1.90(3)-1.94(1) 1.84(4)-1.96(3) Fe-B 1.70(2)-1.85(1) 1.849-1.883(9) N/A C-C N/A N/A 1.29-1.31

7.10 Selected bond lengths for Ce4FeAlC4,Ce4FeGa1-xAlxC4,and C7Fe2C9

Bond Ce4FeAlC4 Length Å Ce4FeGa1-xAlxC4 Length Å Ce7Fe2C9 Length Å Ce-Ce 3.3084(4)-3.7504(6) 3.3465(3)-3.8358(4) 3.4576(5)-3.7465(3) Ce-Fe 2.951(1)-3.3564(5) 3.2832(7)-3.3229(4) 3.0558(9)-3.2957(9) Fe-Fe N/A N/A 2.499(1) C-Fe 1.865(5) 1.871(4) 1.876(5)-2.010(5) C-C 1.355(7) 1.358(6) 1.407(7)

7.2.4 Thermal Analysis TGA/DSC analysis was performed using a TA Instruments Q600 to investigate the melting point and stability of the product. Approximately 2mg of crystals from the reaction were loaded into an alumina cup and then placed into the instrument; argon gas was flowed over the sample at 100mL/min to prevent oxidation of the sample during heating. The sample was heated at 10°C/min to 900°C from 25°C. It was subsequently cooled from 900°C to 100°C at 5°C/min. Powder diffraction data were taken to identify the thermal decomposition products. A mini eutectic flux reaction was performed to determine the temperature of formation of the phase. Reactants were scaled down to a net mass of 150mg to accommodate the instrumental limitations. The same conditions were used as those used to understand the thermal degradation products except the maximum temperature was 950°C.

7.3 Results and Discussion 7.3.1 Synthesis

Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 was synthesized from a reaction of Fe, Al, C, and B in Ce/Co eutectic or Ce/Ni eutectic in a steel crucible. In table 7.11, the different eutectic and reactants

used to isolate Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 in higher yield are listed. The heating profile for all of these reactions are listed in section 7.2.1. As noted in this table, the amount of C and B remain consistent and the concentrations of Al and Fe reactant change.

Table 7.11 Variations in reactant ratio to grow and isolate Ce31.0(2)Fe11.9Al6.5(6)B13C4 in higher yield. Eutectic Reactants Formula Unit

Ce/Co B/C (1:1) Ce31.7(5)Fe10.5(1)Al5C19

Ce/Co B/C (1:1) Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4

Ce/Co Al/Fe/C/B (0.5, 0.5, 1, 1) Ce31.2(1)Fe11.8(5)Al6B13C5

Ce/Ni B/C (1,1) Ce31.4Fe11.1Al6.4C17

Along with these products, the pseudobinary phase of CeCoxFe1-x is observed. This phase crystallizes as octahedron which is easily distinguished from the rod shaped crystals of the title phase. Crystals of the title phase were separated from side products through careful EDS analysis and x-ray diffraction. Elemental composition on crystals of the title phase ranged from Al 1-3 atm%, Fe 15-20 atm%, and Ce 78-82 atm%. In addition to the pseudobinary phase, additional side products of crystallized Ce and Al metal were observed. Although no cobalt incorporation was observed in the EDS data, trace amounts of Fe/Co

mixing may occur in Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 structure. In efforts to eliminate this possibility, reactions were carried out in the Ce/Fe eutectic, which melts at 592°C. Due to the higher melting point, these reactions were centrifuged at 700°C instead of 600°C. Reactions were attempted in both steel and alumina crucibles. Only reactions containing C yielded a new phase

of Ce7Fe2C9. Unfortunately, these crystals were small. Growing larger crystals of this phase for additional characterization was attempted, by increasing the amount of C or Fe, but failed to produced well formed crystals of this phase. Rather, these reactions yielded un-reacted C powder.

Since Ni has been used as a reactant to aid the solubility of B in other systems such as

Ce33Fe14B25C34, the eutectic was changed to Ce/Ni instead of Ce/Co. As discussed previously, this eutectic yielded crystals of the desired phase. Different reaction parameters were attempted in an effort to increase the yield. It was thought that lowering the maximum temperature of the reaction to 850°C and maintaining the slow cooling rate. This would result in a higher yield and larger crystals. Instead of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, a new related phase Ce5(Fe/Ni)3C4, was observed. Elemental analysis of Ce5(Fe/Ni)3C4 showed incorporation of Ni at approximately 17 atm% which suggests that it is stabilized by Ni. An outline of the reactions attempted along with their products is shown in Figure 7.1. These phases contain similar building blocks and will be discussed further in section 7.3.2

Figure 7.1 Outline of reactions attempted in Ce/Ni eutectic

A mini-eutectic flux reaction of Ce/Ni eutectic, C, B, Al, and Fe, was carried out in the TGA/DSC in and attempt to understand the formation of this phase. In figure 7.2, the powder

99 diffraction pattern along with the heat loss curve is shown. Several different exotherms were observed at approximately 900ºC, suggesting that multiple phases were formed as the sample was heated. While the reaction cooled, a large exotherm at 600°C was observed. Powder XRD of this ingot showed peaks corresponding to Ce, Fe2B, Ce7Ni3, AlB2, CeC, Ni, AlNi, Fe19Ni,

FeNi, Fe, Fe7Ni3, and AlB10, phases. There are two possible descriptions for what is occurring in this reaction. One, the formation of this phase is kinetically favored, and it is dependent on how well the reactants mix during heating, or the products on this phase are formed in such low yield that they are below the 5% detection limit of the powder x-ray diffractometer. A stoichiometric synthesis of this phase was attempted and resulted in no formation of the phase. This further suggests that the phase is kinetically stabilized. Based on these results, a larger scale reaction was attempted with reactants of Fe, Al, C, and B, in a 1:1:1:1 mmol ratio and 1.5 g of eutectic. The typical heating profile was adjusted to account for the observed data in the TGA/DSC experiment. Reactions were heated from room temperature to 950°C, cooled to 850°C in 12 hours, annealed at this temperature for 48hrs and slowly cooled to 500°C over the course of 84 hrs. Ideally, this reaction should have resulted in more kinetically stabilized phases because faster cooling rates should quench the reaction. This experiment resulted in crystals of poor quality that could not be accurately identified by single crystal x-ray diffraction.

Figure 7.2 TGA/DSC analysis of a mini-eutectic flux reaction of Ce/Ni eutectic. Powder X-ray diffraction analysis of the resulting ingot showed that multiple binary phases are formed during this reaction suggesting that Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 is a small constituent of the overall reaction.

100

Synthesis of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5, a Pr zeolite-like structure, was accomplished using Pr/Ni eutectic flux. It was initially synthesized with reactants of B and C. Advantageous leaching of Fe and S from the steel resulted in a low yield of this phase. These crystals grew much larger than their Ce counterparts, especially when S was intentionally added to the reaction, and were a lot easier to characterize.

Since S increased the size and yield of the products of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5, reactions were carried out with Ce instead. No crystals of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 were observed in these reactions. This resulted in no incorporation of S and only crystallized Ce metal and CeCo1-xFex were observed in these reactions. It is interesting to note that an analog of

Ce5(Fe/Ni)3C4was not observed in these reactions, which suggests that trace amounts of Al along with Ni are required to stabilize that phase. This suggests that the smaller S atom stabilizes

Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 and Al atom stabilizes the Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 structure, which is more difficult to form. Boron and carbon are difficult to distinguish in a structure. A combination of synthesis, bond length analysis, and elemental characterization was used to determine whether or not these elements were present in these structures. Syntheses with or without C and B present resulted in binary phases. Only when both elements were present in the reactions did these phases form. Bond length analysis for these compounds is useful because the B-Fe bond length should be shorter than that of the C-Fe bond length as shown in table 7.9. XPS analysis can help confirm the presence of these elements. Below, in figure 7.3 is the XPS data from a single crystal of

Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 that was previously screened by single crystal x-ray diffraction.

Figure 7.3 XPS data of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 showing the presence of both a) B and b) C in the sample.

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7.3.2 Crystal Structure

Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 crystallizes in the centrosymmetric space group Pmmm, with a unit cell of a = 11.3888(1) Å b = 15.580(1) Å and c = 15.581(1) Å (R1 = 0.0394, Z=2). This structure features ~ 9 Å zeolite like cages which are composed of planar FeC3 units and Fe-Fe bonds having a length of 2.82(2) Å connecting these rings together. This bond length is similar to that of alpha-Fe, which has a bond length of 2.539 Å-2.931 Å [84]. The Fe-C bond lengths range from 1.85(1) Å – 1.96(2) Å. This compares well with 1.815 Å - 2.213 Å for Fe-C bonds found in Fe3C [124] and Fe5C2 [125].

Along both a- and b- axes, Al atoms are coordinated to 9 surrounding Ce atoms to form monocapped square anti-prisms. Ce-Al bond lengths in this compound range from 2.965(5) Å- 3.469(9) Å. This compound compares well with the bond lengths of 3.136 Å-3.407 Å observed in CeAl [126]. The shorter bond length of 2.965(5) Å may be caused by the mixing of Al and Fe on the same site. Bond lengths between Fe and Ce range from 2.923(2) Å – 3.554(1) Å and Ce-Ce bond distances are 3.139(9) Å–3.871(1) Å. Although the length of 3.139(9) Å is shorter than expected, it has been observed in α-Ce [127] and the high temperature variations of this structure [128] (Figure 7.4 and 7.5).

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Figure 7.4: Blue spheres represent Al atoms, yellow Ce atoms, red Fe atoms, black C/B atoms. Polyhedra are drawn to depict the coordination environment around the Al and Fe atoms. a) Unit cell representation of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 the a/b plane b) Al coordination by 9 Ce atoms, resulting in a square capped anti-prism c) coordination environment of trigonal planar FeC3 units that compose the zeolite-like cages.

Figure 7.5: Blue spheres represent Al atoms, yellow Ce atoms, red Fe atoms, black C/B atoms. a) Ce atoms removed to show the zeolite like cages that run along the c-axis. b) Ce atoms removed to show the zeolite like structure with the bridging Fe-Fe bonds in the b/c plane. c) representation of the FeC3 cage around the square capped anti-prism.

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There is a very small phase width associated with this compound. When a reactant ratio of ½: ½:1:1 mmol of Fe, Al, C, and B was used, Ce31.2(1)Fe11.8(5)Al6B13C5was observed.

Ce31.2(1)Fe11.8(5)Al6B13C5 (R1 = 0.0395 Z = 2) crystallizes in the centrosymmetric space group Pmmm, with a unit cell of a = 11.3888(8) Å , b = 15.580(1) Å c = 15.581(1) Å. The difference between this structure and the one above is the differences in occupancies and Wyckoff sites in the compounds. In Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, the Fe2 (4(z)) site is partially occupied at 95.095% and the Fe3 (4(x)) site is 96.772% occupied. These sites in are fully occupied

Ce31.2(1)Fe11.8(5)Al6B13C5. Ce9 and Ce10 are partially occupied (99.117% and 89.289% respectively) and the partial occupancies in Ce31.2(1)Fe11.8(5)Al6B13C5 are different at 96.853% and 45.056% respectively. There are also differences between atom occupancies on Wykoff sties. In the Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 structure the 2(o) and 2(n) sites are occupied by Ce12 and Al4 respectively but in Ce31.2Fe11.89Al6B13C5 structure, Al4 and Ce13 occupy those sites. Al5 is mixed with Fe5a on the 2(l) site in Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4 but in Ce31.2(1)Fe11.8(5)Al6B13C5, there is no mixing of Fe on that site. Finally, partial occupancy of the 1(c) site occurs in

Ce31.2(1)Fe11.8(5)Al6B13C5at 54.370% and an additional Ce site Ce19, is observed. In both structures, solving in the Tetragonal space group P4/mmm was attempted, but there was a large Q peak that could not be placed in the structure. This suggested that P4/mmm was not the proper space group assignment and lower symmetry was required to solve the structure. The subtle differences in the b- and c- axes were important for assignment of the proper space group even though the axes are within experimental error of each other. The crystallographic structure appears to be dependent on the initial stoichiometry of the reaction. Less disorder is observed in Ce31.2(1)Fe11.8(5)Al6B13C5and less mixing between Al and

Fe is observed. Obtaining the final structure solution of Ce31.2(1)Fe11.8(5)Al6B13C5was complicated by the negative thermal parameters observed for light elements with anisotropic refinement. Isotropic refinement of these sties resulted in positive thermal parameters. This may have been due to light elements being surrounded by large heavy atoms.

Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 crystallizes in the centrosymmetric space group Pmmm with unit cell parameters of a = 11.387(1) b = 15.635(2) c = 15.637(2) (Z = 2, R1 = 0.0305). This structure, like Ce31.2Fe11.89Al6B13C5, has ~9 Å zeolite like cages composed of trigonal planar

FeC3 units. Sulfur is coordinated by 9 Ce atoms which fill the center of the FeC3 cages.

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Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 has 19 crystallographically distinct Pr sites, and four of them are partially occupied: (2s (70.3%), 1g (81.9%), 1c (96.0%), and 1a(80.4%)). There is no site mixing between Pr and Fe or S and Fe (Figure 7.6).

Figure 7.6: Blue spheres represent S atoms, green Pr atoms, red Fe atoms, black C/B atoms. Polyhedra are drawn to depict the coordination environment around the Al and Fe atoms. a) Unit cell representation of Pr31.6(5)Fe10.4(4)S6.6(4)B12C5 in the a/b plane b) Al coordination by 9 Pr atoms, resulting in a square capped anti-prism c) coordination environment of trigonal planar FeC3 units that compose the zeolite-like cages.

Reactions that were carried out in Ce/Ni eutectic resulted in the formation of the zeolite like phase and the structurally related phase Ce5(Fe/Ni)3C4. This related phase crystallizes with a new structure type in the orthorhombic space group Pmc2/1 with unit cell parameters a =

5.272(1) Å, b = 20.417 Å, and c = 13.829(3) Å (Z=2; R1 = 0.0262). This phase contains an unusual asymmetric FeC5 unit. Instead of trigonal planar units observed in La3.67[Fe(C2)3] [122] and La11(MnC6)3 [67] where the transition metal is coordinated by either 3C2 or 3C6 units, the bond lengths associated with the FeC5 units are; 1.84(4)-1.96(3)Å for Fe-C bond and 1.29 Å - 1.31 Å for C-C bonds(Figure 7.7). The terminal C is coordinated by 6 Ce atoms to form a distorted octahedron.

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Figure 7.7 The black spheres represent C, the brown spheres Fe and yellow spheres Ce. This structure was synthesized in the same reaction as the Ce zeolite like structure. a)Unit cell representation of Ce5(Fe/Ni)3C4. b) Coordination environment around the terminal C atom of the trigonal planar FeC5 units. c) Coordination environment of the FeC5 unit.

Ce7Fe2C9 crystallizes in the centrosymmetric space group P21/c (centrosymmetric space group #14) with unit cell parameters a =8.7632(5) Å, b= 7.4926(5) Å, c=10.5016(5) Å, and β =

121.457(4)° with R1 = 0.0189 and Z = 2. This phase was synthesized from Ce/Fe eutectic flux in alumina crucibles with only C as a reactant. This structure contains Fe-Fe dimers surrounded by C atoms. Bond lengths between C and Fe are 1.876(5) Å -2.010(5) Å and Fe-Fe bond lengths 6- are 2.499(1) Å. These structural units are similar to the FeC4 polyanion of [129] except these are discrete (Figure 7.8 and 7.9)

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Figure 7.8 Unit cell representation of Ce7Fe2C9. The black spheres represent C, the brown spheres Fe and yellow spheres Ce. Polyhedra of C and Fe atoms are shown.

Figure 7.9 Coordination environment of Fe2C8 clusters in Ce7Fe2C9. The black spheres represent C, the brown spheres Fe and yellow spheres Ce. a) Fe2C8 clusters surrounded by Ce atoms. b) Fe2C8 structural unit with Fe-Fe bond of 2.499(1)Å.

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As reactions were explored to increase the yield of Ce31.0(2)Fe11.8(5)Al6.5(6)B13C4, a new structure, Ce4FeAlC4, was synthesized. Ce4FeAlC4 crystallizes in the centrosymmetric space group Pcca (space group #54) with unit cell parameters a = 5.7560(7) Å, b = 8.297(1) Å, and c = 15.476(1) Å. Bond lengths between the Fe and C are 1.865(5) Å and C-C bond lengths are 3- 1.355(7) Å-1.358(6) Å, forming FeC4 units. This is similar to the NiC2 polyanionic network observed in CeNiC2 [12] except these FeC4 units are discrete (Figure 7.10). Since this phase forms readily, Ga was added to these reactions. A similar, but lower space group product was formed. Ce4FeGa1-xAlxC4 (x = 0.15) crystallizes in the centrosymmetric space group C2/m (space group #12) with unit cell parameters a = 5.7843(5) Å, b = 15.359(1) Å, and c = 8.2809(7) Å. The bond length between Ce1-Ga1 is 3.2394(6) Å and compares well with the expected length of

3.180 Å -3.783 Å observed in Ce3Ga2 [130].

Figure 7.10 The black spheres represent C, the brown spheres Fe, blue spheres Ga/Al, and yellow spheres Ce. a) Unit cell representation of Ce4FeGa1-xAlxC4 (x= 0.15,1.0) b) Ce and Ga/Al spheres removed and the Fe/C layers can be seen. c) The surrounding environment of the Fe/C chain can be seen. By adding additional unit cells, the structural units of FeC4 are observed.

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The crystallographic structures of these compounds are slightly different, owing to the size of main group element incorporated. When Al is directly added to the synthesis of

Ce4FeGa1-xAlxC4, a higher yield with larger crystals is obtained. This suggests that Al helps to stabilize this phase. When Ga is not present, the same structure is formed but with higher symmetry.

7.4 Conclusion This project was an opportunity to explore the effect of reaction temperature, reactant concentration, and crucible choice and has opened up a new vein of Ce chemistry. Further optimization of these phases may lead to new magnetic behavior and could answer questions regarding the geometry of magnetic ions and their interactions. The unique orientation of 9Å channels that are composed of Fe and the clusters of Al@9Ce, may align and result in cooperative behavior.

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Chapter Eight

Conclusions and Future Work

8.1 A Promising direction

A new structure, Ce10Co2B7C16, was synthesized from Ce/Co eutectic flux in alumina crucibles with reactants boron and carbon. Starting materials were stored and handled in an argon-filled dry-box. Powders of carbon black carbon black (95-97% Strem Chemicals) and boron (99% Strem Chemicals) were used in synthesis. Ce/Co eutectic was synthesized from Ce ingot with Co slug (99% Alfa Aesar) and was combined in a 76:24 [6] atomic ratio. The eutectic was synthesized by arc melting their respective elements on a water-cooled copper hearth into a button that was turned over several times and re-melted to ensure homogeneity of the flux material. The eutectic was broken up into approximately 1mm3 pieces. Reactants were sandwiched between layers of eutectic flux with more on the top than the bottom. This allows reactants that have lower densities to mix with the eutectic as it melts. Once the crucible was loaded with the desired reactants and placed into a quartz tube, a piece of silica wool was used as a filter for centrifugation. The silica tube containing the alumina crucible was fused with the reactants and eutectic were fused under a vacuum of 10-2 Torr. The ampoule was heated to 950°C in 3 hours, held at this temperature for 12 hours, and then cooled to 850°C in 10 hours. The reaction mixtures were subsequently annealed for 48 hours at 850°C and then cooled to 600°C in 84 hours. At 600°C the ampoule was removed from the furnace, quickly inverted, and placed into a centrifuge to decant the molten flux.

Ce10Co2B7C16 crystallizes in silver, thin, flake-like crystals that have no well defined geometric shape. Elemental analysis on single crystals of these compounds showed a composition of approximately 6% Al, 17% Co, and 76% Ce. The Al may be leached from the crucible, but single crystal refinement suggests that Al is not present. Attempts were made to substitute Fe on the Co sites in the structure. Fe was added to reactions in 0.1mmol increments. Powder x-ray diffraction and EDS analysis was used to determine the products of the reaction.

Past 0.1 mmol, Ce10Co2B7C16 was not observed in any of the reactions. Phases such as

Ce33Fe14B25C34 and other byproducts were observed (Figure 8.1).

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Figure 8.1 Powder diffraction pattern of Ce10Co2B7C16 along with the powder diffraction patterns of those reactions containing Fe. Only peaks associated with the desired phase Ce10C2B7C16 were observed at 0.1 mmol.

Preliminary single crystal data on Ce10Co2B7C16 suggests that this phase crystallizes in the P-1 space group with unit cell parameters of a = 8.500(8) Å, b = 8.505(8) Å, c = 13.56(1) Å,

α = 93.65(1)°, β = 100.97(1)°, and γ = 90.03(1)° (R1 = 0.0542, Z = 4). This structure features 1D squares of Co surrounded by chains of C and B (figure 8.2 and figure 8.3). The single crystal refinement of this phase is not very stable. After checking for higher symmetry and possibly twinning of the structure, it appears that P-1 best describes this structure. However, even in the low symmetry space group, there are still negative thermal parameters. This may be attributed to defects in the structure or possible split sites.

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Figure 8.2 Structure of Ce10Co2B7C16. Yellow atoms represent Ce, blue atoms Co, black atoms C, and grey atoms B. a) Unit cell representation. b) Two carbon atoms surrounded by 6 Ce atoms in the center of the unit cell, are represented by black polyhedral. C) Unit cell with bonding removed except those associated with C/Co cluster.

Figure 8.3 Yellow atoms represent Ce, blue atoms Co, black atoms C, and grey atoms B. a) C/Co cluster with C/Co bonds. Angles between each of the Co atoms is 90°. Each side of the Co square capped by a C atom. b) view of the Co/C unit down the c-axis with an extended view of the C/Co cluster.

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Magnetic measurements on single crystals of this phase were dominated by Pauli- paramangetic behavior and a moment per Cerium atom was calculated to be 2.70µB with a Weiss constant of 101.72 (Figure 8.4). The large Weiss constant suggests there is ferromagnetic coupling between the magnetic ions. There is no observed ferromagnetic transition in the temperature dependent measurements. Field dependent measurements at 2K suggest that there is some ordering of the magnetic ions at this temperature. There is no saturation of the magnetic moment and the moment per Cerium atom in the field dependence data is 0.56µB. This suggests there are competing interactions between both the Ce and the Co or the 10 different Ce sites in this compound (Figure 8.5).

Figure 8.4 Temperature dependent susceptibility measurements of Ce10Co2B7C16. Fitting of the high temperature data resulted in a calculated moment per Ce of 2.57µB. At low temperatures, a Curie tail is observed.

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Figure 8.5 Field dependence data at 2K and 200K. At 2K, the moment per Ce is very low, but a small hysteresis was observed. It is possible that there are competing moments between the 10 Ce sites and Co. At 200K the field dependence data resulted in paramagnetic behavior which is consistent with the temperature dependent susceptibility data.

8.2 Conclusion In this research Ce/Co, Ce/Fe, and Ce/Ni eutectics were used. These fluxes were a useful synthetic media to explore new thermodynamically and kinetically stabilized phases. Changes in reaction stoichiometry, elemental form, and crucible choice resulted in different phases. Prediction of the magnetic behavior of these compounds is difficult because these compounds contain isolated networks of carbon anions.

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Future work on these types of compounds should focus on a deeper understanding of how structure correlates to magnetic behavior. Mössbauer spectroscopy was used to understand the local environment of the Fe clusters in Ce33Fe14B25C34, but greater understanding of the mechanism of magnetic coupling in these structures may be achieved with neutron diffraction.

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References

[1] Phelan WA, Menard MC, Kangas MJ, McCandless GT, Drake BL, Chan JY. “Adventures in crystal growth: synthesis of single crystals of complex intermetallics.” Chemistry of Materials 24 (2012): 409-420.

[2] Kanatzidis MG, Pöttgen R, Jeitschko W. “The Metal Flux: A Preparative Tool for the Exploration of Intermetallic Compounds.” Angewandte Chemie 44 (2005): 6996-7023.

[3] Canfield PC and Fisk Z. “Growth of Single Crystals From Metallic Fluxes.” Philosophical Magazine B 65 (1992): 1117-1123.

[4] Chemical Rubber Company: Handbook of Chemistry and Physics Web Edition 2011-2012

[5] Fisk Z, Remeika J.R.Handbook of the Physics and Chemistry of Rare Earths, G. Schneider and Eyring. Vol. 12(1989) Chapter 8.

[6] Massalski T.B., Okamoto H., Binary Alloy Phase Diagrams. 2nd. Ed., ASM International, Materials Park, OH, 1990.

[7] Massalski T.B., Okamoto H., Binary Alloy Phase Diagrams. 2nd. Ed., ASM International, Materials Park, OH, 1990 (on-line ed.)

[8] Dashjav E, Kreiner G, Schnelle W, Wagner FR, Kniep R, Jeitschko W. “Ternary rare earth and actinoid transition metal carbides viewed as carbometalates” Journal of Solid State Chemistry 180 (2007): 636-633.

[9]Cromer DT, Larson AC Roof RB Jr. “The Crystal Structure of UMoC2.” Acta Crystallographica 17 (1964): 272-276.

[10]Reehuis M, Zeppenfeld K, Jeitschko W, Stüsser, Ouladdiaf B, Loidl A. “Magnetic phase transitions in the ternary carbides Ln2Cr2C3 (Ln= Tb, Ho, Er).” Journal of Magnetism and Magnetic Materials 251 (2002): 260-270.

[11]Jeitschko W, Block G, Kahnert GE, Behrens RK. “Pr2ReC2 and other ternary rare erath metal rhenium carbides with filled PbCl2 (Co2Si) structure.” Journal of Solid State Chemistry 89 (1990): 191-201.

[12]Pecharsky VK, Miller LL, Gschneider KA. “Low temperature behavior of two ternary lanthanide nickel carbides: superconducing LaNiC2 and magnetic CeNiC2.” Physical Review B 58 (1998): 497.

[13]Jeitschko W, and Block G. “Ternary Carbides Ln2Mn17C3-x (Ln = La, Ce, Pr, Nd, Sm) with Filled Th2Zn17 Type Structure.” Inorganic Chemistry 25 (1986): 279-282.

[14]Yakel HL. “The Crystal Structure of a Boron-Rich Boron Carbide.” Acta Crystallographica B31 (1975): 1797.

116

[15]Babizhetskyy V, Hiebl K, Mattausch H, Simon A. “New ternary boride carbides RE10B9+xC10-x RE = Gd, Tb, x≈ 2 Infinite boron carbon branched chains.” Zeitschrift für und Allgemeine Chemie 636 (2010): 1229-1235.

[16]Babizhetskyy V, Hiebl K, Mattausch H, Simon A. “Crystal chemistry and physical properties of the ternary compounds REBC (RE = Ce, Pr, Nd).” Solid State Sciences 11 (2009): 501-506.

[17]Babizhetskyy V, Hiebl K, Kremer RK, Mattausch H, Simon A “Crystal Chemistry and physical properties of the ternary compounds R10B9C12 (R = La, Ce, Pr, Nd).” Solid State Sciences 9 (2007): 1126-1134.

[18]Babizhetskyy V, Mattausch H, Simon A. “Kristallstruktur der neuen ternären Boridcarbide SE2BC (SE=Pr, Nd).” Zewitschrift für Anoroganische und Allgemeine Chemie 632 (2006): 2115.

[19]Massalami M. El, Costa S, Rapp RE, Chaves FAB “Chracterization of RCo2B2C (R = rare earth, Y): Phase stability, nonsuperconductivity, and breakdown of the de Gennes scaling)” Physical Review B 62 (2006) 8942-49

[20]Baggio-Dartovitch E.M., Sánchez D.R., Micklitz H. “Magnetic and Structural Features of RNi2B2C and RNiBC (R = Er, Ho, Dy, Tb, Gd) superconducting compounds. Brazillian Journal of Physics 32 (2002) 739-743

[21] Leng Y Materials Characterization: Introduction to Microscopic and Spectroscopic Methods John Wiley and Sons 2008

[22] Gabriel BL SEM: A User’s Manual for Materials Science Materials Park, OH 1985

[23] Kotani A and Ogasawara H “Theory of Core-level Spectroscopy of rare-earth oxides” Journal of Electron Spectroscopy and Related Phenoma 60 (1992) 257-299

[24] Watts JF and Wolstenholme J An Introduction to Surface Analysis by XPS and AES 2nd ed. 2003.

[25] Wagner C.D., Naumkin A.V., Kraut-Vass A., Allison J.W., Powell C.J., Rumble J.R. NIST X-ray Photoelectron Spectroscopy Database Version 3.5; National Institute of Standards and Technology: Gaithersburg, 2003; http://srdata.nist.gov/xps.

[26] West A. R. Basic Solid State Chemistry 2nd ed. John Wiley & Sons, Ltd.: Chichester, West Sussex, England, 1999

[27] Cotton FA Chemical Applications of Group Theory 3rd ed. John Wiley & Sons, Ltd.: New York 1990

[28] Bilbao Crystallographic Server http://www.cryst.ehu.es

117

[29] L Lingwei, Nishimura K., Fujii M, Matsuda K, Huo D “Effects of Tm substitution on superconductivity and magnetism in the antiferromagnetic borocarbide superconductor Dy1-xTmxNi2B2C” Physical Review B 81 (2010) 214517-1-6

[30] Dickson DPE and Berry FJ ed. Mössbauer Spectroscopy Cambridge University Press: New York, 1986

[31] Long GL ed. Mössbauer Spectroscopy Applied to Inorganic Chemistry Vol 1. Plenum Press: New York, 1984

[32] Bo-ping, Coey JMD “Crystal structure, magnetism and 57Fe Mössbauer spectra of ternary RE6Fe11Al3 and RE6Fe13Ge compounds” Journal of Magnetism and Magnetic Materials117 (1992) 225-231

[33] Fyfe CA Solid State NMR for Chemists Ontario Canada 1983.

[34] Bruch MD NMR Spectroscopy Techniques 2nd ed. Practical Spectroscopy Series Vol 21. Marcel Dekker 1996

[35] Sandor MT, Kecskes LJ, He Q, XU J, Wu Y “Correlation of mechanical properties in bulk metallic glasses with 27Al NMR characteristiscs” Chinese Science Bulletin 56 (2011) 3937-3941

[36] Tobash, P.H., Meyers J.J, DiFillippo G., Bobev, S., Ronning F., Thompson J.D., and Sarrao J.L “Structure and Properties of a New Family of Nearly Equiatomic Rare-Earth Metal- Tin-Germanides RESN1+xGe1-x (RE = Y, Gd – Tm): an Unusual Example of Site Preferences Between Elements from the Same Group” Chemistry of Materials 20 (2008) 2151-2159.

[37] Fokwa B.P.T., Misse P.R.N., Gilleβen M., Dronskowski R “Sn-flux syntheses, characterization and bonding analyses of OsB and TiB2” Journal of Alloys and Compounds 489 (2010) 339-342.

[38] Ni N., Bud’ko S.L., Kreyssig A., Nandi S., Rustan G.E., Goldman A.I., Gupta S., Corbett J.D., Kracher A., Canfield P.C. “Anisotropic thermodynamic and transport properties of single-crystalline Ba1-xKxFe2As2 (x =0 and 0.45)” Physics Review B 78 (2008) 014507.

[39] Mathieu J.L., Latturner S.E., “Zintl phase as dopant source in the flux synthesis of Ba1- xKxFe2As2 type superconductors” Chemical Communications (2009) 4965-4967.

[40] Dybkov V.I., Khoruzha V.G., Sidorko V.R., Meleshevich K.A., Samelyuk A.V., Berry D.C., Barmak K “Interfacial interaction of solid cobalt with liquid Pb-free Sn-Bi-In-Zn- Sb soldering alloys” Journal of Materials Science 44 (2009) 5960-5979

[41] Gao F., Cheng F., Nishikawa H., Takemoto T “Characterization of Co-Sn intermetallic compounds in Sn3.0Ag0.5Cu09.5Co lead-free solder alloy” Materials Letters 62 (2008) 2257-2259

118

[42] Lang A., Jeitschko W. “Two New Phases in the System Cobalt-Tin: The Crystal Structures of α- and β-CoSn3,” Z. Metallkd. 87 (1996) 759-764

[43] SAINT, version 6.02a; Bruker ACS, Inc,: Madison, WI, 2000

[44] Sheldrick G.M, SHELXTL NT/2000, version 6.1, Bruker AXS, Inc., Madison, WI, 2000

[45] Macaluso R.T., Millican J.N., Nakatsuji S., Lee H.O., Carter B., Moreno N.O., Fisk Z., Chan J.Y. “A comparison of the structure and localized magnetism in Ce2PdGa12 with the heavy fermion CePdGa6” Journal of Solid State Chemistry 178 (2005) 3547-3553

[46] Benbow E.M., Dalal N.S., Latturner S.E “Crystal growth and magnetic behavior of R6T13- xAlxMy phases (R = La, Nd; T = Mn, Fe; M = main group) grown from lanthanide-rich eutectic fluxes” Journal of Solid State Chemistry 182 (2009) 3055-3062

[47] Nesmeyanov A.N. Vapor Pressure of the Chemical Elements., Elsevier, Amsterdam, 1963.

[48] Villars, P., Calvert L.D. Pearson’s Handbook –Crystallographic Data for Intermetallic Phases, Materials Park, OH, ASM International, 1998.

[49] Schreyer M., Kraus G., Fassler T.F. “Novel Combination of Coordination Polyheadra in Mg2Co3Sn10+x—The First Phase in the Mg/Co/Sn System” Zeitschrift für anorganische und allgemeine Chemie 630 (2004) 2520-2526

[50] Zhong G., Lei X., Mao J “Chemical bonding, electronic, and magnetic properties of R3Co4Sn13 intermetallics (R = La, Ce, Sm, gd, and Tb): Density functional calculations” 79 (2009) 094424

[51] Vaqueiro P., Sobany G.G., “A Powder Neutron Diffraction Study of the Metallic Ferormagnet Co3Sn2S2” Solid State Sciences 11 (2009) 513-518

[52] Szytula A., Leciejewicz J., Handbook of Crystal Structures and Magnetic Properties of Rare Earth Intermtallics., CRC Press, Boca Raton, FL., 1994.

[53] Latturner S.E., Kanatzidis M.G. “RE(AuAl2)nAl2(AuxSi1-x)2 A New Homologous serius of Quaternary Intermetallics Grown from Alumina Flux” Inorganic Chemistry47 (2008) 2089-2097.

[54] Whalen J.B., Zaikina J.V., Achey R., Stillwell R., “Metal to Semimetal Transition in CaMgSi Crystals Grown from Mg-Al Flux” Chemistry of Materials 22 (2012) 1846-1853

[55] Sieve B., Chen X., Cowen J., Larson P., Maharti S.D., Kanatzidis M.G. “Multinary Intermetallics from Molten Al: Synthesis of SmNiAl4Ge2 and YNiAl4Ge2. Possible Spin Frustration in Separated Triangular Sm3+ Layers” Chemistry of Materials11 (1999) 2451

[56] Benbow E.From Paramgnetism to Spin Glasses: Magnetic Studies of Single Crystal Intermetallics. Ph.D. dissertation 2008. Florida State University

119

[57] Atoji M, Williams DE “Neutron Diffraction Studies of La2C3, Ce2C3, Pr2C3, and Tb2C3” Journal of Chemical Physics 35 (1961)

[58] Boucherle J.X., Givord F., Lejay P., Schweizer J., and Stunault A. “Structures of Ce2Sn5 and Ce3Sn7, two superstructures of CeSn3” Acta Crystallographica B 44 (1988) 377-380

[59] Zachariasen WH “Crystal chemical studies of the 5f series of elements” Acta Crystallographica 2 (1949) 94-99

[60] Tung LD, Franse JJM, Buschow KHJ, Moze O “Crystal structure and the magnetic properties of the compound CeCoAl4” Journal of Alloys and Compounds 256 (2997) 45- 47

[61] a] Benbow E.M., Dalal N.S., Latturner S.E “Spin Glass Behavior of Isolated, geometrically Frustrated Tetrahedra of Iron Atoms in the Intermetallic La21Fe8Sn7C12” Journal of American Chemical Society 131 (2009) 3349-3354 [b] Zaikina J.V., Schellenberg I., Benbow, E.M., Pöettgen R., Latturner S.E. “Influence of the La/M network on magnetic properties of Mn4 tetrahedra in intermetallic compounds La21Mn8M7C12 (M = Ge, Sn, Sb, Te, Bi)” Chemistry of Materials 23 (2011) 1768-1778

[62] Sologub O.L., Salamakha P.S., Alleno E., Godart C. “Structure and magnetism of ternary compounds Ce5MGe2 M = Co, Ni, Ru, Rh, Pd, Ir, Pt” Journal of Magnetism and Magnetic Materials 226-230 (2001) 170-172

[63] Stojanovic M., Latturner S.E. “Europium substitution into intermetallic phases grown in Ca/Zn flux.” Journal o f Solid State Chemistry 182 (2009) 2239-2245

[64] Reynolds P.C., Stojanovic M., Latturner S.E. “Flux growth of new cobalt-zinc-tin ternary phase Co7+xZn3-xSn8 and its relationship to CoSn” Journal of Solid State Chemistry 184 (2011) 1875-1881

[65] Lang D. A., Zaikina J.V., Lovingood, D.D., Gedris T.E., Latturner S.E “Ca2LiC3H: A new complex carbide hydride phase grown in metal flux” Journal of American Chemical Society 132 (2010) 17523-17530

[66] Zaikina J.V., Jo Y., Latturner S.E “Ruthenium intermetallics grown from La-Ni flux: Synthesis, structure, and physical properties” Inorganic Chemistry 49 (2010) 4486-4490

[67] Zaikina J.V., Zhou H., Latturner S.E “Structural relationships between new carbide La14Sn(MnC6)3 and fully ordered La11(MnC6)3” Journal of Solid State Chemistry 183 (2010) 3987-2994

[68] Kirchmayr H.R., Poldy C.A. “Magnetism in rare earth—3d intermetallics” Journal of Magnetism and Magnetic Materials 8 (1978) 1-42

[69] Reehuis M., Brown P.J., Jeitschko W., Möller M.H., Vomhof T. “A neutron diffraction study of the magnetic order in the ThCr2Si2 type phosphides PrCo2P2 and NdCo2P2” Journal of Physics and Chemistry of Solids54 (1993) 469-475

120

[70] Tappe F., Hermes W., Eul M., Pöttgen R. “Mixed verium valence and unusual Ce-Ru bonding in Ce23Ru7Cd4” Intermetallics 17 (2009) 1035-1040

[71] Mishra R., Hermes W., Rodewald U.C., Hoffman R.D., Pöttgen R “Trivalent-Intermediate Valent Cerium Ordering in Ce2RuZn4” Zeitschrift für anorganische und allgemeine Chemie 634 (2008) 470-474

[72] Hegger H., Petrovic C., Moshopoulou E.G., Hundley M.F., Sarrao J.L., Fisk Z., Thompson J.D. “Pressure-Induced Superconductivity in Quasi-2D CeRhIn5” Physical Review Letters 84 (2000) 4986-4989

[73] Witte A.M., Jeitschko W. “Preparation and crystal structure of the isotypic carbides Ln3.67TC6 (Ln = rare earth elements; T = Mn, Fe, Ru) and Eu3.16NiC” Zeitschrift für naturforschung B 51 (1996) 249-256

[74] Sieve B., Gray D.L., Henning R., Bakas T., Schultz A.J., Kanatzidis M.G. “Al Flux Synthesis of the Oxidation-Resistant Quaternary Phase REFe4Al9Si6 (RE = Tb, Er)” Chemistry of Materials 20 (2008) 6107-6115

[75] McCallum R.W., Fisher I.R., Anderson N.E., Canfiled P.C., Kramer M.J., Dennis K.W. “Reentrant behavior in the temperature dependence of metamagnetic transitions in single crystal Nd6Fe13-xAl1+x” IEEE Transactions on Magnetics 37 (2001) 2147-2149

[76] Szytula A., Leciejewicz J., CRC Handbook of Crystal Structures and Magnetic Properties of Rare Earth Intermetallics. CRC Press: Boca Raton, FL, 1994

[77] Davaasuren B., Bormann H., Dashjav W., Kreiner G., Widom M., Schnelle W., Wagner F.R., Kniep R. “Planar Fe6 Cluster Units in the Crystal Structure of RE15Fe8C25 (RE = Y, Dy, Ho Er)” Angewandte Chemie International Edition 49 (2010) 5688-5692

[78] Villars P., Calvert L.D. Pearson’s Handbook –Crystallographic Data for Intermetallic Phases ASM International: Materials Park, OH, 1998.

[79] Peter S.C., Malliakis C.D., Nakotte H., Kothapilli K., Rayaprol S., Schultz A.J., Kanatzidis M.G. “The polygallides: Yb3Ga7Ge3 and YbGa4Ge2” Journal of Solid State Chemistry, 187 (2012) 200-207

[80] Salvador J.R., Bilc D., Mahanti S.D., Kanatzidis M.G. “Stabilization of β-SiB3 from Liquid Ga: A Boron-Rich Binary Semiconductor Resistant to High-Temperature Air Oxidation” Angewandte Chemie International Edition 42 (2003) 1929-1932

[81] Gerss M.H., Jeitschko W., Boonk L., Nientiedt J., Grobe J., Morsen E., Leson A. “Preparation and crystal structure of superconducting Y2feC4 and isotypic lanthanoid iron carbides” Journal of Solid State Chemistry 70 (1987) 19-28

[82] Shoemaker C.B., Shoemaker D.P., Fruchart R. “The Structure of a new magnetic phase related to the sigma phase: iron neodymium boride Nd2Fe14B” Acta Crystallographica C. 40 (1984) 1665-1668

121

[83] Anderson S., Carlsson J.O. “The Crystal Structure of MnB4” Acta Chemica Scandinavica 24 (1970) 1791-1799

[84] Teplykh P.A., Pirogov A.N., Kuchin A.G., Teplykh A.E. “Real crystal structure and magnetic state of Ce2Fe17 compounds.” Physica B: Physics of Condensed Matter 350 (2004) E99-E102

[85]Shoemaker C.B., Shoemaker D.P., Hopkins T.E., Yindepit S. “Refinement of the structure of β-manganese and of a related phase in the Mn-Ni-Si system” Acta Crystallographica B 34 (1978) 3573-3576

[86] Babizhetskyy V., Hiebl K., Kremer R.K., Mattausch H., Simon A. “Crystal Chemistry and Physical Properties of the Ternary Compounds Ln10B9C12 (Ln: La, Ce, Pr, Nd)” Solid State Sciences 9 (2007) 1126-1134

[87] Bidaud E., Hiebl K., Hoffmann R.D., Pöttgen R., Jardin C., Bauer J., Gautier R., Goougeon P., Saillard J.Y., Halet J.F. “Structural, Electronic, and Magnetic Properties of Ternary Rare-Earth Metal Borocarbides R5B2C5 (R=Y, Ce-Tm) Containing BC2 Molecules” Journal of Solid State Chemistry 154 (2000) 286-295

[88] Babizhetskyy, V., Hiebl, K., Mattausch, H., Simon, A. “Crystal chemistry and physical properties of the ternary compounds RE5B4C5 (RE = Ce, Pr, Nd)” Journal of Physics and Chemistry of Solids 70 (2009) 561-566

[89] Babizhetskyy V., Mattausch H., Simon A., Hiebl K., Yahia M. B., Gautier R., Halet J.F. “New examples of ternary rare-earth metal boride carbides containing finite boron-carbon chains: The crystal and electronic structure of RE15B6C20 (RE = Pr, Nd)” Journal of Solid State Chemistry 181 (2008) 1882-1890

[90] VanDuijn J., Suzuki K., Attfield J.P. “Boron-Carbon Order in CeB2C2” Angewandte Chemie International Edition 39 (2000) 365-366

[91] Jardin C., Oeckler O., Mattausch H.J., Simon A., Halet J.F., Sallard J.Y., Bauer J. “Synthesis, Characterization, Structural and Theoretical Analysis of a New Rare-Earth Boride Carbide: Lu3BC3” Zeitschrift für anorganische und allgemeine Chemie 627 (2001) 1389-1394

[92] Jardin C., Oeckler O., Mattausch H.J., Simon A., Halet J.F., Sallard J.Y., Bauer J. “Synthesis, Characterization, and Structural and Theoretical Analysis of Gd4B3C4: A Novel Rare Earth Metal Borocarbide Containing Two Different Boron-Carbon Arrangements” Inorganic Chemistry 39(2000) 5895-5900

[93] Lassoued S., Gautier R., Boutarfaia A., Halet J.F. “Rings and chains in solid-state metal borides and borocarbides. The electron count matters” Journal of Organometallic Chemistry 695 (2010) 987-993

122

[94] Renaudin G., Dieudonne B., Avignant D., Mapemba E., El-Ghozzi M., Fleutot S., Martinex 3+ 4+ H., Cerny R., Dubois M. “Pseudotetragonal Structure of Li2+xCex Ce12-x F50: The First Mixed Valence Cerium Fluoride” Inorganic Chemistry 49 (2010) 686-694

[95] Beche E., Charvin P., Perarnau D., Abanades S., Flamant G. “Ce 3d XPS investigation of cerium oxides and mixed cerium oxide (CexTiyOz)” Surface and Interface Analysis 40 (2008) 264-267

[96] Goremychkin E.A., Osborn R., Sashin I.L., Riseborough P., Rainford B.D., Adroja D.T., Lawrence J.M. “The Transition from Heavy Fermion to Mixed Valence in Ce1-xYxAl3: A Quantitative Comparison with the Anderson Impurity Model” Physical Review letters 104 (2010) 176402

[97] Schmitt R., Blaschkowski B., Eichele K., Meyer H. “Calcium tetraboride – Does It Exist? Synthesis and Properties of a Carbon-Doped Calcium Tetraboride that is Isotypic with the Known Rare Earth Tetrabordies” Inorganic Chemistry 45 (2006) 3067-3073

[98] Zogal O.J., Florian P., Massiot D., Paluch S., Shitsevalova N., Borshchevsky D.F. “ 11B and 89 Y MAS NMR and magnetic characterization of YB4” Solid State Communications 149 (2009) 693-696

[99] [a] Han M.K., Morosan E., Canfield P.C., Miller G.J. “The coloring problem in intermetallicas: bonding and properties of Tb3Zn3.6Al7.4 with the La3Al11 structure type” Zeitschrift für Kristallographine 220 (2005) 95-101 [b] Gout D., Benbow E., Gourdon O., Miller G.J, “Theoretical studies on cerium nickel aluminides: polar intermetallics with heavy fermion behavior” Journal of Solid State Chemistry 176 (2003) 538-548

[100] Kittel C. Introduction to Solid State Physics, 6th ed,; John Wiley & Sons, Inc.: New York, 1986.

[101] Palstra T.T.M., Nieuwenhuys G.J., Mydosh J.A., Buschow K.H.J.: “Mictomagnetic, ferromagnetic, and antiferromagnetic transitions in La(FexAl1-x)13 intermetallic compounds” Physics Review B. 31 (1985) 4622-4632

[102] Blanchard P. E. R., Cavell R.G., Mar A. “Effects of rare-erath subsititution in the oxyarsenides REFeAsO (RE = Ce, Pr, Nd, Sm, Gd) and CeNiAsO by X-ray photoelectron and absorption spectroscopy” Journal of Solid State Chemistry 183 (2010) 1477-1483

[103] Journet C., Maser W.K., Bernier P., Loiseau A., Chapelle M., Lefrant S., Deniard P., Lee R., Fisher J.E. “Large Scale production of single-walled carbon nanotubes by the electric arc technique” Letters to Nature 388 (1997) 756-758

[104] Jia Z., Wang Z., Liang J., Wei B., Wu D. “Production of short muti-walled carbon nanotubes” Carbon 37 (1999) 903-906

[105] Baughman R.H., Zakhidov A.A., Heer W.A. “Carbon Nanotubes – the route towards applications” Science 297 (2002) 787-792

123

[106] Dresselhaus M.S., Dresselhaus G., Jorio A., Filho A.G., Saito R. “Raman spectroscopy on isolated single wall carbon nanotubes” Carbon40 (2002) 2043-2061

[107] Oncel C., Yurum Y., “Carbon Nanotube Synthesis via the Catalytic CVD Method: A Review on the Effect of Reaction Paramters” Fullerenes, Nanotubes and Carbon Nanostructures 14 (2006) 17-37

[108] Tessonnier J.P. Su D.S. “Recent Progress on the Growth Mechanism of Carbon Nanotubes: A Review” Chemistry and Sustainability, Energy & Materials 4 (2011) 824- 847

[109] See C.H., Harris A.T. “A review of carbon nanotube synthesis via fluidized-bed chemical vapor deposition.” Industrial Engineering chemical Research 46 (2007) 997-1012

[110] Bhaviripudi S., Mile E., Steiner S.A., Zare A.T., Dresselhaus M.S., Belcher A.M., Kong J. “CVD Synthesis of Single-Walled Carbon Nanotubes from Gold Nanoparticle Catalysts” Journal of the American Chemical Society 129 (2007) 1516-1517

[111] Ghorannevis Z., Kato T., Kaneko T., Hatakeyama R. “Narrow-Chirality Distributed Single-Walled Carbon Nanotube Growth from Nonmagnetic Catalyst” Journal of the American Chemical Society 132 (2010) 9570-9572

[112] Gao X.P., Qin X., Wu F., Liu H., Lan Y., Fan S.S., Yuan H.T., Song D.Y., Shen P.W “Synthesis of carbon nanotubes by catalytic decomposition of methane using LaNi5 hydrogen storage alloy as a catalyst” Chemical Physics Letters 327 (2000) 271-276

[113] Zhao H., Bradford P.D., Wang X., Liu W., Luo T.J., Jia Q., Zhu Y., Yuan F.G. “Intermetallic Fe-Zr catalyst used for growing long carbon nanotubes arrays” Materials Letters 64 (2010) 1947-1950

[114] Ma X., Chen B., Latturner S.E. “Synthesis and Properties of New Multinary Silicides R5Mg5Fe4AlxSi18-x(R = Gd, Dy, Y, x ≈ 12) Grown in Mg/Al Flux” Inorganic Chemistry 51 (2012) 6089-6095

[115] Tucker P.C., Nyffeler J., Chen B., Ozarowski A., Stillwell R., Latturner S.E. “A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare- Earth/Transition metal Eutectic Fluxes” Journal of American Chemical Society 134 (2012) 12138-12148

[116] Dresselhaus M.S., Dresselhaus G., Saito R, Jorio A. “Raman Spectroscopy of Carbon Nanotubes” Physics Reports 409 (2004) 47-99

[117] Jorio A., Pimenta M.A., Filho A.G.S., Saito R., Dresselhaus G., Dresselhaus M.S. “Characterizing carbon nanotube samples with resonance Raman scattering” New Journal of Physics 5 (2003) 139

124

[118] Datsyuk V., Kalyva M., Papagelis K., Parthenios J., Tasis D., Siokou A., Kallitsis I., Galiotis C. “Chemical oxidation of multiwalled carbon nanotubes” Carbon 40 (2008) 833-840

[119] Davaasuren B., Borrmann H., Dashjav E., Kreiner G., Widom M., Schnelle W., Wagner F.R., Kniep R. “Planar Fe6 Cluster Units in the Crystal Structure of Re15Fe8C25 (RE = Y, Dy, Ho, Er)” Angewandte Chemie International Edition 49 (2010) 5688-5692

[120] Goerens C., Brgoch J., Miller G.J., Fokwa B.P.T “Scaffolding, Ladders, Chains, and Rare Ferrimagnetism in Intermetallic Borides: Synthesis, Crystal Chemistry and Magnetism” Inorganic Chemistry 50 (2011) 6289-6296

[121] Rotter M., Tegel M., Johrendt D “Superconductivity at 38K in the Iron Arsenide (Ba1- xKx)Fe2As2” Physical Review Letters 101 (2008) 107006

[122] Davaasuren B., Dashjav E., Kreiner G., Bormann H, Kniep R “Reinvestigation and Superstructure of La3.67[Fe(C2)3]” Journal of Solid State Chemistry 182 (2009) 1331

[123] Hoffman RD, Pöttgen R, Jeitschko W “Scandium transition metal carbides Sc3TC4 with T= Fe, Co, Ni, Ru, Rh, Os, Ir” Journal of Solid State Chemsitry (1992) 134-139

[124] Westgren A “Cementitens kristallstruktur, Översikt över föregaende undersökningar", Jernkontorets Ann. 87 (1932) 457–468.

[125] Jack KH, Wide S “Nature of chi-carbide and its possible occurrences in steel” Nature 212 (1966) 248-250

[126] Bede C, Lemaire R “Structures crystallines des composes DyAl et des autres composes equiatomiques de l’aluminium avec les metaux des terres rares” Acta Crystallographica 23 (1967) 840-845

[127] Zachariasen WH “Crystal structure of the alpha Cerium phases” Proceedings of the National Academy of Sciences USA 75 (1978) 1066-1067

[128] Zachariasen WH Proceedings of the National Academy of Sciences USA 75 *1978) 75

[129] Kahnert G, Jeitschko W “The carbides RE2MnC4 (RE = Y, Er, Tm) with Er2FeC4- type structure”

[130] Yatsenko SP, Grin YU, Schevich OM, sabirsianov NA, Fedorchuk AA “Kristallstruktur von “Ce3Ga2”” Journal of the Less Common metals 160 (1990) 229-235

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Biographical Sketch

Education

The Florida State University

University of Toledo B.S. in Chemistry and B.A. in Psychology graduation with honors both degrees

Research Experience

The Florida State University, 2009-present Advisor Dr. Susan Latturner Graduate student in the Inorganic Division. Research project involves working with metal fluxes for the growth of new magnetic materials using the Ce/Co eutectic. Oak Ridge National Lab SULI Intern 2007 Advisor Dr. Anatoli Melechko Undergraduate research project involved working on understanding ideal growth conditions for nanofibers as supports for lipid bilayers. Lipids were synthesized and added to a hydrophobic nanofiber surface in hopes of the lipids stretching themselves across the nanofiber support. Other projects involved patterning surfaces for various applications such as understanding toxicity of nanomaterials.

University of Toledo Undergraduate Researcher 2008 Advisor Dr. Dean Giolando Undergraduate research project involved understanding the effects of polymers on filling holes and improving electrical conductivity of solar cells.

University of Toledo Undergraduate Researcher 2007 Advisor Mr. James Zubricki Undergraduate research project in modeling biological proteins. Used RASMOL, a modeling software to produce models and send them for manufacture.

University of Toledo Undergraduate Researcher 2006 Advisor Dr. Jared Anderson Undergraduate research project involved ionic liquids and understanding their properties through gravimetric analysis.

Employment

Perstorp Polyolys Lab Tech 2006-2008 Supervisor Mr. Phillip Blosser

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QA/QC in process and final testing of manufactured product to fit customer specifications. Final testing of producing included GC, HPLC, UV-vis, titrations, general wet chemistry techniques as well as participation in safety ISO EHS14001audits.

Technical Experience

Synthesis: RF furnace, flux growth techniques, arc melting, chemical vapor deposition, physical vapor deposition, reactive ion etching, optical lithography

Characterization: Single crystal and powder x-ray diffraction; optical spectroscopy (UV-vis, fluorescence, Raman spectroscopy); surface characterization, (Atomic Force Microscopy (liquid and ambient conditions), surface profilometry, magnetic measurements (SQUID, Mössbauer spectroscopy); elemental analysis (EDS, XPS); thermal analysis (SDT); Gas Chromatography, HPLC; scanning electron microscopy (SEM)

Teaching experience University of Toledo Undergraduate TA 2008 Supervisor Dr. Alice Skeens Prepared lectures for class as well as led two recitation sections. The Florida State University Grader Fall 2008 Honors Organic Chemistry 1 Supervisor Dr. Dudley Grader Spring 2009 Honors Organic Chemistry 2 Supervisor Dr. Dudley TA Summer 2009 Organic Chemistry Lab Supervisor Dr. Hilinski TA Fall 2009 Organic Chemistry 2 Supervisor Dr. Dudley TA Spring 2010 Organic Chemistry 2 Supervisor Dr. Kearley TA Summer 2010 Organic Chemistry Lab Supervisor Dr. Sal Profeta TA Fall 2010 Organic Chemistry Lab Supervisor Dr. Sal Profeta

Presentations and Posters

“Synthesis, Structure, and Characterization of the new intermetallic compound

Nd2Co2SiC” FIMS October 2011 “Studies of Ce rich carbide and borocarbide materials” North American Solid State Conference June 2011 Poster Presentation

“Growth of Pr19Fe18SC8B from Pr/Co flux” FAME, Inorganic Symposium May 2011 “Studies of Ce rich carbide and borocarbide materials” Inorganic Division Prospectus March 2011 “Metal Flux Synthesis of CoZnSn” FAME, Inorganic Symposium May 2010 “Mn2+ doped GaAs for Spintronic Applications” Inorganic Division Seminar April 2010 “Metal Flux Synthesis of CoZnSn” Inorganic Division Seminar September 2009

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“Synthesis and Integration of Carbon Nanofibers for Biological Applications” Undergraduate poster session, Oak Ridge National Labs, April 2008 Publications

Tucker PC, Lita A, Latturner SE “CVD Growth of Carbon of Carbon Nanotubes using Iron- containing Intermetallics as Catalysts” In Preparation

Wang M., Tucker PC, Latturner SE “Synthesis of Nd2Co2SiC from Nd/Co eutectic flux”In Preparation

Tucker PC, Nyffeler J, Chen B, Ozarowski A, Stillwell R, and Latturner SE “A tale of two metals: New cerium iron borocarbide intermetallics grown from rare-earth/transition metal eutectic fluxes” Journal of American Chemical Society 134 (2012) 12138

Reynolds PC, Stankovich M, Latturner, SE “Flux growth of a new cobalt-zinc-tin ternary phase Co7+xZn3-xSn8 and its relationship to CoSn” Journal of Solid State Chemistry 184 (7) 2011

Awards

Undergraduate Creativity Award- Biological Modeling Congress of Graduate Students- Travel Award

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