New Materials Grown from Ca/Li Flux David A

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2011 New Materials Grown from Ca/Li Flux David A. Lang

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COLLEGE OF ARTS AND SCIENCES

NEW MATERIALS GROWN FROM CA/LI FLUX

DAVID A. LANG

A Thesis submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirement for the degree of Master of Science

Degree Awarded: Summer 2011

The members of the committee approve the thesis of David Lang defended on June 22, 2011.

______Susan Latturner Professor Directing Thesis

______Albert E. Stiegman Committee Member

______Oliver Steinbock Committee Member

Approved:

______William Cooper, Associate Chair, Department of Chemistry and Biochemistry

The Graduate School has verified and approved the above-named committee members.

TABLE OF CONTENTS

List of Tables iv

List of Figures v

Abstract vi

Chapter 1: Introduction to flux synthesis 1

1.1 Metal flux 1 1.2 Alkaline earth/alkali metal fluxes 2 1.3 Expected products of synthesis in Ca/Li flux 5 1.3.1 Zintl phases 5 1.3.2 Intermetallics 6 1.3.3 Hydrogen storage materials 7 1.4 Characterization of products 7 1.4.1 Elemental analysis 8 1.4.2 X-ray diffraction 8 1.4.3 NMR 9 1.4.4 Protolysis, hydriding and dehydriding 10 1.4.5 Band structure calculations 10

Chapter 2: LiCa2C3H 11

2.1 Introduction 11 2.2 Experimental procedure 11 2.3 Results and discussion 15

Chapter 3: LiCa7Si3H3 23

3.1 Introduction 23 3.2 Experimental procedure 23 3.3 Results and discussion 25 3.4 Thermo-volumetric measurements 31

Chapter 4: LiCa7Ge3H3 and LiCa11Ge3OH4 32

4.1 Introduction 32 4.2 Experimental procedure 32 4.3 Results and discussion 35 4.4 Future work 40

Chapter 5: Conclusions and future work 41

5.1 Ca/S/C 41 5.2 Ca/B/C 43

References 45

Biographical sketch 48

iii

LIST OF TABLES

2.1. Data collection and structure refinement parameters for LiCa2C3H 13 2.2. Atomic coordinates, site occupancy factors (s.o.f.), and thermal

displacement parameters for LiCa2C3H 13

2.3. Selected interatomic distances (Å) in the LiCa2C3H structure 14

3.1. Crystallographic data collection parameters for LiCa7Si3H3 24

3.2. Atomic position and thermal displacement parameters for LiCa7Si3H3 24

3.3. Bond lengths observed for LiCa7Si3H3 25 4.1. Selected crystal data and structural refinement parameters for

LiCa7Ge3H3 and LiCa11Ge3OH4 34 4.2. Atomic coordinates and thermal displacement parameters for

LiCa7Ge3H3 34 4.3. Atomic coordinates and thermal displacement parameters for

LiCa11Ge3OH4 35

4.4. Comparison of selected interatomic distances in LiCa7Ge3H3 and

LiCa11Ge3OH4 37

LIST OF FIGURES

1.1. A representative depiction of metal crucibles and their contents after centrifugation 2 1.2. The Ca/Li phase diagram. The circle highlights the 50:50 Ca/Li mixture and its melting point of 300°C 3 1.3. Heating profile commonly used for Ca/Li flux reactions 4 1.4. Calcium distillation apparatus 5 1.5. Vial on the right contains calcium metal before distillation; vial on the left is filled with Ca after distillation 5

2.1. A crystal of LiCa2C3H under paratone oil 16

2.2. Powder X-ray diffraction patterns for products of Ca/Li/C/CaH2 reactions with reactant mmol ratios of 10:10:6:x 16

2.3. Structure of LiCa2C3H 17 7 2.4. Li MAS NMR spectra of LiCa2C3H samples synthesized using

Ca/Li/C/CaH2 mmol ratios of 10:10:6:x 20 2.5. 13C MAS-NMR spectrum 20 1 2.6. H MAS-NMR spectrum of LiCa2C3H 21

2.7. Total and partial density of states for LiCa2C3H 22

3.1. SEM micrograph of a crystal of LiCa7Si3H3 26

3.2. Structural variants of Ca3Ni3Si2 26

3.3. Structure of LiCa7Si3H3 27

3.4. View of hydride layer in LiCa7Si3H3 (viewed down the b-axis), and

coordination environment of the three hydride sites 28

3.5. MAS NMR spectra of LiCa7Si3H3 30

3.6. Hydride decompositions of LiAlH4 standard material are shown by

the 2 spikes in pressure that are marked 31 4.1. Powder X-ray diffraction of Ca/Li/Ge/CaH2/CaO reactions. 36

4.2. The LiCa11Ge3OH4 structure, viewed down the c-axis 38

4.3 The LiCa11Ge3OH4 structure, highlighting the locations and connectivity of the hydride-centered octahedra (yellow) 39

4.4. Density of states diagram for LiCa7Ge3H3 40

5.1. Structure of Ca12S(C3)2(CxH1-x)4 42

5.2. Structure of (Ca/Li)15(CBC)6(C2)2(C)x 44

ABSTRACT

Molten metal fluxes are useful in materials synthesis. Dissolution in molten metal activates reactants at temperatures well below their melting point. Lower temperatures and the modified energetics in a flux increase the possibility of isolation of complex metastable or kinetically stabilized phases. In contrast to the powders often obtained by conventional solid state synthesis methods, the solution phase character of flux reactions promotes the growth of crystals, which are required for accurate structural and electronic characterization. Ca/Li flux solvates lightweight refractory elements such as carbon and boron. It also dissolves many ionic compounds such as CaH2, Ca3N2, and LiF. This allows for synthesis of phases ranging from strongly delocalized intermetallics to complex salts. In particular, dissolution of CaH2 makes Ca/Li flux a very promising medium for growth of new metal hydride phases. These hydride compounds may have great potential as hydrogen storage materials. The ionic products such as Zintl and complex salt phases have interesting electronic properties because these classes of compounds are usually semiconductors.

Reactions of light main group elements (B, C, Si) and ionic compounds (CaH2, CaO, Li3N) have been carried out in Ca/Li flux. Several Zintl phase hydrides including LiCa2C3H and LiCa7Si3H3 have been produced. These compounds are of interest because of their new crystal structures and their release of H2 gas in decomposition reactions. Structures are discussed in detail to explain the different bonding environments in these compounds. Efforts to explore and eliminate the oxide and hydride contaminants in the reactive flux have also netted interesting results. A new boride carbide was found using distilled flux.

CHAPTER 1

FLUX SYNTHESIS

1.1 Metal flux synthesis A major contribution to advances in the study of intermetallic and Zintl phases is the development of new synthetic methods. Traditionally, reactants are combined in specific ratios to target specific phases. The reactants are ground and pressed into a pellet, which is heated at very high temperatures. The product of such reaction is generally isolated in powder form and rarely pure after one cycle; several cycles of grinding and heating are often required. The main disadvantage of this synthetic method is that it favors thermodynamically driven products, which are often binary phases or simple ternary phases. As an alternative to traditional methods, an appropriate flux can be carefully chosen to dissolve reactants at much lower temperatures than typically used for solid-state synthesis.1,2,3 This will enhance diffusion and favor kinetically stable products because it allows reactants to find a low temperature product with local energy minimum structure. This will eliminate unwanted thermodynamically driven products like binary phases and simple ternary phases. Additionally, the liquid state of reaction will promote the growth of crystals making the products easier to analyze and molten flux is in many cases easily removed leaving the pure product. For a metal to be a good choice as a flux for reaction chemistry it should melt and dissolve reactants at reasonably low temperatures so that normal heating equipment and containers can be used, and it must allow separation of the excess flux from the products. Additionally, it is important for the reactants to have reasonable solubilities in the flux to avoid exceedingly long growth times.4 While many metals have very high melting points, eutectic combinations of different metals often have melting points well below the temperatures of classical solid-state synthesis. In a reactive flux components act not only as solvents, but also as reactants, providing species that can be incorporated into the final product.5 Flux and reactants are combined in a crucible, which is placed into a fused silica tube and sealed under vacuum. The reaction tubes are placed in a furnace and the heating profile is chosen based on the melting point of the flux and the solubility of the reactants in it. The temperature needs to be somewhat elevated to dissolve the refractory elements, such as carbon. When the reactions are done excess flux is removed, while it is still liquid, by centrifuging and the product is isolated (see figure 1.1).

Figure 1.1. A representative depiction of metal crucibles and their contents after centrifugation. Crystalline products adhere to the reaction end of crucible (left), and flux melt is decanted to the other end (right).

The crucible material is determined by the flux that is being used. A crucible should be resistant to corrosion by the solvent. As a general rule suitable crucible materials have different bonding from that of the flux solvent and the other reactants that are used in the synthesis. A large difference in the chemical bond type leads to sufficiently small solubility of the crucible material in the flux solvent, especially when the material has a high melting point. For instance, platinum crucibles are often recommended for oxide melts, since the ionic reactants will not attack the Pt vessel. Additionally, strength of the crucible needs to be high enough to contain pressures from the solution and any gas present or generated at the high temperatures at which the synthesis takes place. Welded steel or niobium crucibles will resist corrosion by alkali and alkaline earth flux, and will withstand the vapor pressure generated when these volatile metals are heated.

1.2 Alkaline earth/alkali metal fluxes Ca/Li flux solvates lightweight refractory elements such as carbon and boron. It also dissolves many ionic compounds such as CaH2, Ca3N2, and LiF. This allows for synthesis of phases ranging from

strongly delocalized intermetallics to complex salts. In particular, dissolution of CaH2 makes Ca/Li flux a very promising medium for growth of new metal hydride phases, which are of great recent interest as hydrogen storage materials. The binary phase diagram for Ca/Li shows the effect of reducing the melting temperature as increasing proportions of Li are added (Figure 1.2).6 All of the products formed in this flux are Ca rich so Ca is usually disproportionately precipitated out of the flux during crystal growth. Therefore, maintaining a sufficient proportion of Ca while still having a workable flux melting temperature is important. The 50/50 proportion mixture works well by supplying an ample amount of Ca while still having a melting point below 400°C. Reactions are maintained at temperatures well above this to ensure that the flux does not become too viscous which would inhibit reactant diffusion and centrifugation of the solvent.

Figure 1.2. The Ca/Li phase diagram. The circle highlights the 50:50 Ca/Li mixture and its melting point of 300°C.13

Crucible materials are important because they both contain and interact with the flux reaction. Welded metal crucibles are essential to the Ca/Li flux syntheses because both Ca and Li will vaporize at elevated temperatures if left in an open crucible. Welded steel tubes are the most widely used crucible material for Ca/Li flux. Occasionally, contaminants from the steel may be an issue and niobium tubing is used as a substitute. Sealed reactions are heated to 1050°C and then slowly cooled to the centrifuge temperature of 500°C (see Figure 1.3). Reactions need to be centrifuged wile the flux is still liquid so that the crystal product may be retrieved.

1200

1000

800

600

400

Temp celcius Temp 200 0 0 50 100 150 200 Hours

Figure 1.3. Heating profile commonly used for Ca/Li flux reactions.

Alkaline earth metals obtained from chemical suppliers have a significant amount of hydrogen incorporation. Extensive research by the Corbett group has indicated that inadvertent hydride incorporation is common in the synthesis of alkaline earth-rich phases; careful synthetic investigations

on reported phases Ba5Ga6, Ba21Ge2O5, and the A5Sb3 (A = Ca, Sr, Ba) family showed that these 7,8,9 compounds are actually Zintl phase hydrides (Ba5Ga6H2, Ba21Ge2O5H24, and A5Sb3H). The yield of these phases is greatly improved when hydride is deliberately added. 9 A distillation device was designed to remove hydrogen from the shelf supply of Ca. This was prepared using a custom built steel vessel with a water-jacketed vacuum attachment; this was attached to a diffusion pump system (Figure 1.4). Several grams of commercial Ca shot was placed in the steel vessel and heated to 600-620°C under a dynamic vacuum of 10-4-10-5 Torr to decompose the calcium

hydride contaminant and remove the resulting hydrogen gas. The hot H2 reacts with any calcium oxide coating on the metal to form water vapor which is also removed by the vacuum. The calcium was held under these conditions for ~4 hours (until the vacuum level fell below 10-5 Torr and maintained the low pressure, indicating the end of hydrogen production), then cooled. This is not technically a "distillation"; the calcium does not evaporate and recondense on the cool part of the tube. It is only

heated to a temperature sufficient to decompose the CaH2. However, the phrase "distillation" is used to be consistent with Corbett's work. The "distilled" Ca product appears more reflective and metallic then the commercial Ca (see Figure 1.5). The availability of distilled calcium enables greater control over the growth of hydride phases. The amount of hydride included in the flux reaction is determined by how much

CaH2 is deliberately added as a reactant, instead of how much is present in the contaminated flux metal.

Figure 1.4. Calcium distillation apparatus. Metal is placed in section to the left (within a tube furnace) and vacuum line attachment is to the right.

Figure 1.5. Vial on the right contains calcium metal before distillation; vial on the left is filled with Ca after distillation.

1.3 Expected products of synthesis in Ca/Li flux

1.3.1 Zintl phases Zintl phases are charge balanced compounds that usually consist of a very electropositive component (alkali and alkaline earth metals) and an electronegative component (a p-block metal or metalloid). In terms of extended solids, the boundary of this class of material is not entirely fixed but rather fits between intermetallic compounds and salts. The electronic character of Zintl phases also fits between these classes of materials. Zintl phases are charge-balance due to transfer of all the metal valence e- to the p-block metal/metalloid elements, which gain anionic charge and/or form bonds to achieve an octet. For instance, reaction of potassium with silicon yields K4Si4. In this phase, the

potassium atoms donate their valence e- to silicon atoms which achieve a stable octet by forming bonds 4- to 3 other silicon atoms, forming tetrahedral Si4 anions. Zintl phases are usually semiconductors, as compared to intermetallics being electron delocalized metals and salts that are usually electron localized insulators. These Zintl phase semiconductors have electrons localized on the electronegative elements with delocalization occurring when electrons are excited. Semiconductor materials are the foundation of modern electronics, included in radio, computers, telephones, solar cells, LED’s and many other devices. Intrinsically, Zintl phase semiconductors are of potential interest in the development of new devices. Some Zintl phases feature clusters or chains of light elements such as B, C, or N as anions.10 4- Examples of such compounds are Mg2C3 that features a C3 chain and Ca5Cl3(C2)(CBC) that features a CBC5- chain.11,12 The stricter definitions of Zintl phase state that they should incorporate only the heavier p-block elements, so compounds with carbide anions would instead be considered complex salts. However, in compounds such as Mg2C3, the basic requirements of classification as a Zintl phase are met: the non metals in the anion achieve an octet and have covalent bonding between them, and the alkali or alkaline earth metal remain in an ionic network around the anionic clusters. Li compounds may also stretch the Zintl definition because of the ability of this ion to polarize e- density and form slightly 13 covalent interactions; evidence of this is seen in LiCa2C3H (see chapter 2).

1.3.2 Intermetallic compounds Intermetallic compounds are a class of compounds composed of definite proportions of two or more metals. In contrast to Zintl phases, intermetallics rarely form charge balanced compounds. While a clear cut positive charge on the cation can be observed, quantifying anionic charge is not possible. Rather than electrons predominantly being localized on the most electronegative element, they are delocalized throughout the lattice leading to metallic characteristics. Although anions do not play the same role in intermetallics when compared to Zintl phases, some cluster-like units are found in intermetallics. In the example of La14Sn(MnC6)3, carbon and Mn form propeller like trigonal planar anionic units, although of indeterminate charge.14 Some intermetallic materials have been known to have interesting properties that have found commercial uses. Many examples of intermetallic materials exhibit ferromagnetic ordering like alnico, 2,15 Nd2Fe14B, or FeCo, used as permanent magnets. Niobium-Tin or other binary phases with the A15 16 lattice type are used as superconductors. LaNi5 has hydrogen storage capabilities. Additionally, intermetallic materials have been used as shape memory alloys like Cu-Al-Ni, coating materials like

Nitinol, high temperature structural materials like Ni3Al and dental amalgams, which are alloys of 17,18,19,20 intermetallic Ag3Sn and Cu3Sn.

1.3.3 Hydrogen storage in intermetallics and ionic compounds

The solubility of CaH2 in Ca/Li flux and the observed hydride products may present a new way to synthesize hydrogen storage materials. Studies of promising binary intermetallic hydrogen storage alloys such as LaNi5 and Mg2Ni show the mechanism of hydrogen absorption and groundwork for improving the capacity and behavior of the materials. These hydrogen storage materials are comprised of a very electropositive hydride-forming element A (an element that will combine with H2 to form a stable hydride, such as CaH2) and a weakly-hydride forming element B (a more electronegative transition metal that does not form a stable hydride). The structure of the compound determines the amount of hydrogen that can be taken up, and also the stability and reversibility of the process. A high concentration of A element will increase the hydrogen capacity, but will also increase the likelihood of an irreversible disproportionation due to the high stability of the hydride bond. Hydrogen exhibits the highest heating value per mass of chemical fuels. Additionally, hydrogen can be generated by water splitting methods and it is environmentally friendly. There are several reasons why hydrogen is not the major fuel of today’s energy consumption. The important drawback with hydrogen as an energy carrier is its low critical temperature of 33 K (i.e. hydrogen is a gas at ambient temperature). A solid form of hydrogen can greatly reduce the volume per unit of hydrogen. Metal hydride solids can be produced through oxidation of reactive metals, e.g. Li, Na, Mg, Al, Zn with 21,22 water. Some metal hydrides, such as LiAlH4, allow for retrieval of the H2 gas by decomposition of the material.23 Hydrogen-forming decomposition materials can be activated by heat or chemically. Some hydride materials will thermally decompose at specific decomposition temperatures. Other hydride materials, when protons are chemically provided, will combine their H- unit with added H+ in a chemically induced hydrogen-forming decomposition reaction.

1.4 Characterization A number of techniques are used for the characterization of intermetallic and ionic products of flux reactions. The initial process of characterization consists of elemental analysis, single crystal X-ray diffraction and powder X-ray diffraction to determine the composition, structure, and purity of the product. Elemental analysis of a selection of crystals from each reaction is carried out before single crystal X-ray diffraction. It helps determine if known phases have been made and it gives a general idea about elemental composition of the product crystals, it indicates the presence of multiple phases in the product, and it indicates potential impurities and flux coating. Once a product is identified and the reproducibility is verified, the synthesis can be additionally adjusted to increase the yield and the quality/size of the crystals. Determining the melting point of the crystal using TGA/DSC (Thermogravimetric Analyzer/Differential Scanning Calorimeter) can give useful information for optimizing reaction temperatures. Upon determination of a melting point of a

product adjustments can be made in several areas of the synthesis. The temperature program can be adjusted in terms of changing maximum reaction temperature, the cooling time, the annealing time and the centrifuging temperature. After the product is structurally characterized, and its synthesis optimized, it is tested for physical properties. Most of the time specific properties are targeted before the process of synthesis is started. Obviously, exploratory synthesis is somewhat unpredictable but this can be ameliorated by careful selection of the elements used in synthesis. Intermetallic materials as previously mentioned can exhibit a wide range of interesting properties. Depending on the designed target the products may be tested for magnetic properties, resistivity, heat capacity, hydrogen storage capacity and other characteristics.

1.4.1 Elemental analysis The method that is used for elemental analysis is SEM-EDS (Scanning Electron Microscopy- Energy Dispersive Spectroscopy). This is a qualitative method, which is insufficient by itself to give a precise elemental ratio, but very useful when followed up with more quantitative methods. The instrument is JEOL 5900 scanning electron microscope with energy dispersive spectroscopy (EDS) capabilities. Samples from each reaction are affixed to an aluminum SEM stub using carbon tape. The high energy beam of electrons in the microscope hits the sample and ejects core electrons from the atoms in it. Higher shell electrons relax down to fill the resulting holes, causing emission of characteristic X-rays for each element in the sample. Samples are analyzed using a 30 kV accelerating voltage and an accumulation time of 60 seconds. EDS analysis is a surface technique and will be affected by flux coating the crystals. Better results can be obtained by cleaving the crystals and analyzing the inner surface. The data is given in form of either mass or molar percentage and it can only be considered semi qualitative due to lack of standards. The main limitation of this instrument is that it is not capable of detecting elements lighter than sodium. Light elements such as Li and C are not within the detection limits of the EDS. Carbon and hydrogen can be determined by combustion analysis. Analysis of Li is performed using a Perkin-Elmer Analyst 100 atomic absorption spectrophotometer to detect atomic emission of Li. A weighed sample is dissolved in 2% aqueous HCl, and the solution is diluted to within the linear range of the detector. A quantitative calibration curve is obtained by serial dilution of a Li standard solution (1000 ± 1 ppm Li in 2% HCl, Fisher Chemicals). These solutions are ionized over an acetylene flame, and the lithium emission at 670.8 nm is measured. Comparison of the emission of the standard solutions to that of the sample indicates a mass percent that is then compared to the anticipated stoichiometry.

1.4.2 X-ray diffraction

Single crystal X-ray diffraction is used to determine the exact structure of a compound provided that the quality of the crystal is good enough. The single crystal X-ray diffraction instrument used in this work is the Bruker APEX 2 CCD diffractometer with a Mo-Kα radiation source. Air-sensitive sample crystals are brought out of the glovebox under Paratone oil and are mounted in a cryoloop. For air-stable materials, the crystals are mounted on a small piece of glass fiber using nail polish or epoxy. The fiber is attached to the 18 mm Copper CrystalCap with wax. Optimally, for either the cryoloop or glass fiber method, small spherical pieces of crystals are chosen to avoid twinning and absorption problems. After collection of a partial data set (matrix) to determine the quality of the crystal and its unit cell parameters, a full data set is collected using collection frames from 10-20 seconds depending on the size of the crystal. Additionally, KRYO-FLEX low temperature device makes it possible to also analyze air sensitive materials and look for phase transitions between 90 and 300 K. Processing of the data is accomplished with use of the program SAINT; an absorption correction is applied to the data using the SADABS program. Refinement of the structure is performed using the SHELXTL package which utilizes direct methods to locate heavy atoms in the structure; and difference Fourier maps to locate lighter atoms.24 STRUCTURE TIDY is used to standardize the final atomic coordinates. Powder X- ray diffraction is used in some cases as a qualitative method. This method does not allow for determination of new structures but it can be very useful if the verification of known compounds is required. Additionally, it can be used to check the purity of the product because it will indicate presence of multiple phases in the sample. Analysis of product purity is essential when trying to optimize the synthesis. Finally, the method is used when the product is in powder form or when the quality of crystal is not good enough for single crystal diffraction

1.4.3 Nuclear magnetic resonance NMR is most commonly used to characterize the structure of compounds, but it can also be an invaluable tool to explore the electronic properties of extended solids. The resonance frequency of a nucleus is affected by bonding electrons and conduction electrons. Presence of conduction electrons produces an additional field on the nucleus, resulting in a large paramagnetic shift (Knight Shift). Nuclear resonances of metallic samples often appear at much higher frequencies then insulating samples. NMR data is collected on a Varian Inova 500 wide-bore spectrometer at room temperature. Magic angle spinning (MAS) is used to eliminate the effects of anisotropy in solids; rapid spinning at a specific angle randomizes particle orientation, similar to the tumble effect in solution. For determining the location of chemical shift, LiCl for 7Li and tetramethylsilane (TMS) for 1H, 13C, and Si29 are used as standards. Highly conducting samples are ground with KBr in a drybox to eliminate eddy currents and facilitate spinning of the sample in the magnetic field. The mixtures are packed into either 2.5 mm or

4mm zirconia rotors with airtight screw caps. NMR data is collected at 25 °C at an appropriate spin rate, pulse width, and number of scans. Experimental parameters are determined based on the properties of each material.

1.4.4 Protolysis, hydriding, and dehydriding Anions in Zintl phases can be further characterized by observing their reaction with a proton source such as gaseous HCl or H2O, a process referred to as protolysis. For instance, a compound 4- + 4- featuring isolated Si will react with H (g) to form silane, SiH4. But a compound containing Si4 will form more complex gaseous species. For analysis, the Zintl phase solid is reacted with gaseous source of H+ and the gaseous products analyzed by mass spectrometry. To maintain a dry reaction, HCl gas

(generated from thermal decomposition of NH4Cl) is used as a proton source. Gaseous aliquots are analyzed by injecting them into a HP6890 series GC system coupled to a Hewlett-Packard 5973 mass selective detector. Determination of hydrogen in samples is carried out by thermal decomposition. One method to analyze decomposition is by use of the TGA/DSC. Hydrogen loss by the material can be seen as a weight drop in the thermograph. A significant drawback to this method is the possibility of damaging the TGA/DSC if the hydride sample decomposes and releases volatile reactive metals such as Li or Ca. Another method of hydrogen detection is to observe pressure changes while samples are heated. A custom built sample chamber fitted to a tube furnace is in current use in our lab space. This device, by use of a pressure transducer, records pressures as the air-tight chamber is heated. Samples are loaded by disconnecting rubber o-ring face seals. By using face seals in the opening junction parts of the pressure chamber, air tightness is optimized. Pressure readings from the computer output are matched to temperature settings programmed into the furnace. Spikes in pressure during the heating process are from hydrogen release by the sample. Qualitative data of decomposition temperature can be determined by either DSC/TGA or pressure detection methods. Quantitative data of the amount of hydrogen produced be the sample can be determined by equating the intensity of pressure spikes to the ideal gas law.

1.4.5 Band structure calculations Calculations of density of states (DOS) are performed with the tight binding-linear muffin tin orbitals-atomic sphere approximation TB-LMTO-ASA program package, based on the experimentally determined space group, unit cell dimensions, and atomic coordinates.25 After atomic coordinates are entered into an initial TB-LMTO-ASA program directory, a program file is written within TB-LMTO- ASA program fillies. By entering different commands, a list of calculations determines the electronic structural properties. An output file is created to generate a graphical plot.

CHAPTER 2

LiCa2C3H

2.1 Introduction Metal hydrides include intermetallic hydrides with variable content of chemisorbed hydride interstitials (e.g., LaNi5Hx, where x can vary from 0 to 6); ionic hydrides such as NaH or CaH2 or more complex charge-balanced Zintl phase hydrides with covalently bound or anionic hydrogen; and phases 23 containing covalent metallohydride species such as LiBH4 or NaAlH4. In addition to their potential hydrogen storage capabilities, these compounds often lie on the border between complex salts, classical Zintl phases, and intermetallics and feature interesting structural characteristics. The reaction of carbon 4- and CaH2 in Ca/Li flux produces LiCa2C3H, which has a new structure incorporating both the rare C3 carbide anion and a clearly defined hydride anion. This phase was originally formed in small amounts from reactions of carbon in Ca/Li flux made from commercial (hydride-contaminated) calcium metal.

The initial single crystal X-ray structure refinement indicated a stoichiometry of LiCa2C3, but careful - investigation of residual e density revealed incorporation of hydride. Subsequent reactions with CaH2 deliberately added increased the yield. The high reactivity of carbon and the unusual anion formed in Ca/Li flux is notable. Metal carbides are a relatively sparsely investigated class of compounds, with Jeitschko and Pöttgen responsible for much of the recent discoveries of new phases.26 Most metal carbides feature monatomic 4- 2- carbide anions (C ), with acetylide anions (C2 ) being much less common and more complex carbide 4- 4- anions such as C3 rarer still. The C3 anion can be viewed as deprotonated allene. This allenylide anion, first definitively observed in Sc3C4, is also found in Mg2C3, but the calcium analogue of this 26,27 phase does not exist. The structure of LiCa2C3H may be stabilized by a strong interaction between the hydride anion and neighboring lithium cations, which is evidenced by observed bond lengths, density of states and electron localization function calculations, and 1H NMR data. With the 4- - unprecedented coexistence of the C3 and H anions, LiCa2C3H represents a new lightweight complex salt, and the first ordered metal carbide hydride.

2.2 Experimental procedure Reactants were used as received: calcium shot (99.5% Alfa Aesar), lithium rod (99.8% Strem Chemicals), acetylene carbon black (99.9% Strem Chemicals), and calcium hydride (98%, Alfa Aesar).

Ca/Li/C/CaH2 mmol ratios of 10:10:(1-9):(0-1) produced the desired product, with the highest yield produced using a ratio of 10:10:6:1. Reactants were loaded into steel crucibles: these were welded shut under Ar, sealed in fused silica ampoules under vacuum. All reaction ampoules were heated to 1323 K in 2 h and were kept at this temperature for 2 h. They were cooled to 1073 K in 24 h, and then cooled to 773 K in 108 h. At 773 K, the tubes were removed from the furnace, inverted, and centrifuged to force the excess molten Ca/Li mixture off of the product crystals. Much of the crystalline product adheres to the surface of the steel crucible during growth and centrifugation, so use of a filter to separate the crystals from the flux is not necessary. Crucibles were cut open under an argon atmosphere in a glovebox. The solid product consists of silver reflective rectangular crystals, which oxidize within a few minutes of being exposed to air. Elemental analysis was carried out using SEM-EDS, atomic emission spectrometry, and combustion analysis, The SEM-EDS data indicated only the presence of calcium; no heavy elements from the crucible were incorporated. Analysis of lithium content was accomplished using atomic emission studies; comparison of the emission of the standard solutions to that of the sample indicated a

Li content of 5.6% of the total mass of the sample (5.59% Li expected for the LiCa2C3H stoichiometry). Combustion analyses of carbon and hydrogen on a similarly synthesized sample were carried out by Atlantic Microlab Inc. (Theor. Calcd.: C, 29.03%; H, 0.81%. Found: C, 27.30%; H, 0.75%). While the carbon and hydrogen amounts are slightly lower than calculated (possibly due to flux coating or slight decomposition of the sample during transport), the 3:1 C:H ratio is in agreement with the stoichiometry

LiCa2C3H. For single crystal X-ray diffraction studies, sample crystals were brought out of the glovebox under Paratone oil. Small spheroid pieces cleaved from the larger bulk crystals were mounted in a cryoloop. Single-crystal X-ray diffraction data were collected at 150 K in a stream of nitrogen. The structure was solved in tetragonal space group P4/mbm (No. 127); crystallographic data can be found in Tables 2.1-2.3.

a Table 2.1. Data collection and structure refinement parameters for LiCa2C3H.

Composition Ca2LiC3H Crystal system tetragonal Space group P4/mbm (no. 127) Collection temperature, K 150 Cell parameters a = 6.82360(10) Å c = 3.75180(10) Å V, Å3 174.689(6) Z 2 Density (calc.) 2.360 [g cm–3] µ, [mm–1] 3.00 Data collection range, deg. 4.22 < θ < 47.29 Reflections collected 3191 Independent reflections 475 [Rint = 0.0181] 16 Parameters refined

b R1 0.0151 c wR2 [Fo > 4σFo] 0.0407 R1, wR2 (all data) 0.0165, 0.0407 Largest diff. peak and hole 0.59 and -0.36 [e/Å3] Goodness-of-fit 1.149 aFurther details of the crystal structure determination may be obtained from Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number CSD- b c 2 2 2 2 2 1/2 2 2 2 – 421103. R1 = ∑||Fo|–|Fc||/ ∑ |Fo|. wR2 = [Σ w(Fo – Fc ) / Σ w(Fo ) ] , w = [Σ (Fo ) + (A·p) + B·p] 1 2 2 ; p = (Fo + 2Fc )/3; A = 0.0226 Table 2.2. Atomic coordinates, site occupancy factors (s.o.f.), and thermal displacement parameters for

LiCa2C3H.

a) Atom Wyckoff x/a y/b z/c s.o.f. Ueq site

Ca(1) 4h 0.18142(2) 0.68142(2) ½ 1 0.00552(4) C(1) 4g 0.63735(9) 0.13735(9) 0 1 0.00854(12)

C(2) 2d 0 ½ 0 1 0.00553(13)

Li(1) 2a 0 0 0 1 0.01813(59)

H(1) 2b 0 0 ½ 1 0.01755(715)b) a) Ueq is defined as one third of the trace of the orthogonalized Uij tensor. b) isotropic refinement.

Table 2.3. Selected interatomic distances (Å) in the LiCa2C3H structure.

Bond Distance, Å

Ca(1)-C(1) 2.5647(6) ×2

Ca(1)-C(1) 2.8880(4) ×4

Ca(1)-C(2) 2.56590(11) ×2

Ca(1)-Li(1) 3.1269(1) ×4

C(1)-C(2) 1.3254(9)

C(1)-Li(1) 2.6461(4) ×2

Li(1)-H(1) 1.876(1)

Ca(1)-H(1) 2.502(1)

Thermal properties and protolysis of LiCa2C3H were also investigated. Using a TA Instrument Q600 DSC-TGA system, a large single crystal was placed in an alumina sample pan and heated to 1273 K at 10°/min under flowing argon to observe any phase transitions or decomposition. The protolysis reaction between LiCa2C3H and NH4Cl was carried out in a 100 mL flask sealed with a rubber septum to allow aliquots of gaseous products to be taken by syringe. LiCa2C3H (0.124 g) and NH4Cl (0.214 g, 99.5% Sigma-Aldrich) were added to the reaction flask inside the glovebox. The reaction flask was evacuated and then backfilled with nitrogen. The mixture of two powders was heated under flowing nitrogen to ~470 K using a heating mantle. Gaseous aliquots were analyzed by injecting them into a mass spectrometer. Magic angle spinning (MAS) 7Li, 1H, and 13C NMR data were collected at room temperature. LiCl powder was used as external reference for 7Li and tetramethylsilane (TMS) for 1H and 13C. For the 7 Li NMR study, samples of solid product from several Ca/Li/C/CaH2 reactions of differing stoichiometries were each ground with KBr in a drybox (1:4 sample to KBr ratio by mass) to facilitate spinning of the sample in the magnetic field; the mixtures were packed into 2.5 mm zirconia rotors with airtight screw caps. 7Li NMR data were collected at 25 °C and a 13-15 kHz spin rate. A one-pulse sequence was used with a pulse width of 5 µs and a recycle delay of 15 µs. A total of 400 scans were collected for each spectrum. For the 13C and 1H MAS NMR experiments, only the product from a 13 Ca/Li/C/CaH2 flux reaction of ratio 10:10:6:1 was analyzed. The C spectrum was collected on a sample in a 4 mm rotor spinning at 4 kHz with a pulse width of 4 us and a recycle delay of 300 s; 800 pulses were totaled. The spectrum was fitted using Origin software. The 1H spectrum was collected for a sample in a 4 mm rotor spinning at 8 kHz. The 1H spectrum is the sum of 1600 pulses, each with a pulse width of 5 µs and a recycle delay of 1 s.

Calculations of density of states (DOS) were performed based on the experimentally determined unit cell dimensions and atomic coordinates for LiCa2C3H. The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. The calculation was made for 1056 κ points in the irreducible Brillouin. Integration over the Brillouin zone was made by the tetrahedron method.25 The following radii of atomic spheres were applied for the calculations: for LiCa2C3H, r(Ca) = 3.78 Å, r(Li) = 2.74 Å, r(H) = 1.35 Å, r(C) = 1.45 Å. The basis sets contained Ca(3s, 3d), Li(2s), H(1s), and C(2s, 2p) with Ca(3p), Li(2p, 3d), H(2p, 3d), and C(3d) functions being downfolded.

2.3 Results and discussion

LiCa2C3H grows in a 1:1 Ca/Li melt as silvery, reflective, rod-like crystals and agglomerates. Individual crystals up to 1 mm in length are obtained; see Figure 2.1. This phase is unstable to moisture and air and was therefore stored and handled under argon. When heated under argon, the phase is stable to 950 K, at which point it decomposes with a rapid weight loss. The highest yield is produced from a

Ca/Li/C/CaH2 ratio of 10:10:6:1; the yield is approximately 70% based on carbon. The title phase was initially synthesized in the absence of a hydrogen source. The adventitious hydrogen in Ca/Li/C reactions likely came from the calcium or lithium reactants. These metals were not distilled after purchase; even though they were stored in a glovebox, they can react with trace water to form metal hydroxides and hydrogen gas. The hydrogen gas will then react with the metal to form metal hydride; as a result of this, commercial alkaline earth metals may contain up to 20 at. % hydrogen.28 After XRD structure refinement revealed the presence of the hydride anion in LiCa2C3H (vide infra), calcium hydride was deliberately added to subsequent reactions. Ca/Li/C/CaH2 ratios of 10:10:6:x (x = 0, 0.25, 0.5, 0.75, and 1.0) mmol were used. Powder diffraction of the samples indicates that calcium carbide and calcium hydride are formed as byproducts. CaC2 is the main product in reactions with little or no

CaH2 added; the yield of LiCa2C3H increases as larger amounts of CaH2 reactant are used (see Figure

2.2). Recrystallized CaH2 becomes increasingly prevalent in reactions with large amounts of CaH2. The

CaH2 crystals form as large (1-2 mm length) colorless transparent rods, easily distinguishable from the silver crystals of the title phase. The high quality of these crystals allowed for improved structural characterization for this compound, including refinement of the hydrogen positions. This information is of interest, because previous structural studies on CaH2 have reported incorrect hydride positions or 29 were carried out on powdered samples or on deuterides. LiCa2C3H can also be made from a stoichiometric ratio of reactants, using a heating profile identical to the flux reactions. Product yield of stoichiometric reactions was roughly 30% and of poorer crystal quality than product obtained from flux; a significant amount of CaC2 byproduct is apparent by powder X-ray analysis.

1 mm

Figure 2.1. A crystal of LiCa2C3H under paratone oil.

Figure 2.2. Powder X-ray diffraction patterns for products of Ca/Li/C/CaH2 reactions with reactant mmol ratios of 10:10:6:x. a) No CaH2 added (x = 0). Peaks showing the impurity products of CaLi2 and CaC2 are labeled. b) X = 1 mmol amount of CaH2 in proportion to 3 mmol C added to the reaction. The bottom pattern is dominated by the LiCa2C3H phase. C) Theoretical powder pattern of LiCa2C3H calculated from crystal structure.

LiCa2C3H crystallizes with a new structure type in tetragonal space group P4/mbm, shown in Figure 2.3. The refinement of the positions and occupancy of the non-hydrogen atoms was straightforward. All these sites are fully occupied; the calcium, lithium, and carbon positions correspond 30 to the cesium, amide nitrogen, and azide positions in the Cs2(NH2)N3 structure type, respectively. However, an additional peak in the residual electron density of ~1 e Å3 became apparent in the final stages of the structural refinement. This peak is consistently seen in the single-crystal X-ray diffraction analyses of crystals from several different Ca/Li/C/CaH2 flux reactions. It is located on the 2b Wyckoff

site, a site surrounded by 4 neighboring Ca atoms and 2 neighboring Li atoms; the actual observation of an electron density peak at this site was possible because of these relatively light neighboring atoms. The assignment of this peak as hydrogen was made on the basis of previous reports of adventitious hydride incorporation and from inspection of the bond distances to surrounding atoms. The LiCa2C3H structure can therefore be viewed as a stuffed variant of the Cs2(NH2)N3 structure type. The Ca-H distance of 2.5016(1) Å is longer than the range of 2.30-2.36 Å for Ca-H in CaH2; it is also longer than the sum of ionic radii for these elements (2.24 Å).31 On the other hand, the 1.8759(1) Å distance from the hydride to the two adjacent Li ions is shorter than that in LiH (2.031 Å) and shorter than the sum of ionic radii (1.98 Å). Similarly short Li-H distances of 1.88 Å were reported for Li4Si2H; this strong bonding was surmised to stabilize the ternary hydride phase.32

C a a) Li b

1.874Å 2.500 Å H

2.564 Å Å 1.322

2.884 Å

2.645 Å

2.564 Å 2.564

Figure 2.3. Structure of LiCa2C3H. a) Structure viewed down the c-axis. b) Structure viewed down the a-axis; octahedral interstitial sites of the hydride anions are highlighted in yellow. c) Local 4- environment of the C3 anion.

The occupancy of the hydrogen site was fixed at 100% because it cannot be refined based on X- ray single-crystal data. The possibility of variable hydride content (LiCa2C3Hx, with 0 < x < 1) was

considered; intermetallic hydrides allow for a wide range of hydrogen incorporation (0 < x < 6 for

LaNi5Hx, for example), and small deviations from unity are also reported in more charge-balanced 33 systems such as Ca5Bi3Hx. However, powder and single-crystal diffraction of products of reactions carried out with differing amounts of CaH2 showed none of the variation in unit cell parameters which is typically associated with Vegard’s law behavior. Instead, the unit cell parameters remained constant, but the yield and product quality of the title phase improved as the amount of CaH2 reactant was increased. This indicates that this phase forms in the completely hydrided state, which is also properly charge-balanced. 4- The title compound is one of a very small number of inorganic compounds containing the C3 anion. This chain of three carbons, which can be viewed as deprotonated propadiene (C3H4, also known as allene), is also found in Mg2C3, Sc3C4, R4C7 (R = Y, Ho, Er, Tm, Lu), R5Re2C7 (R = Sc, Er, Tm, Lu), 27,27,34-36 and Ca3C3Cl2. In LiCa2C3H, this anion is perfectly linear, with the central carbon located on a center of inversion. The C-C bond length of 1.3254(9) Å is similar to that in propadiene (~1.31 Å), 4- indicating its double bond nature; the other reported structures containing the C3 unit also feature a C- C bond of similar length (range: 1.32-1.35 Å). The distribution of cations around this unit is shown in

Figure 2.3c. While Mg2C3 and Sc3C4 both feature clustering of cations around the terminal carbons of 4- the C3 anion (in agreement with the location of formal charges), LiCa2C3H, R4C7, and Ca3C3Cl2 have short metal-carbon bonds to the central carbon as well. This may be indicative of the relative ionicity of 4- 2+ the interactions between the metal cations and the C3 unit, with the smaller and more polarizing Mg and Sc3+ inducing a more covalent interaction with the terminal carbon atoms of the carbide anion, as compared to the more ionic interactions with larger cations Ca2+ and R3+. 4- Reaction of a compound containing the C3 unit with a protic reagent should yield gaseous 27,34 C3H4, as was reported for Mg2C3 and R4C7. LiCa2C3H reacts rapidly and violently with water. A more controllable protolysis reaction was carried out using ammonium chloride as the source of acidic protons via its thermal decomposition to gaseous HCl and ammonia, following the procedure of a 27 similar study on Mg2C3. The reaction with LiCa2C3H is shown below:

LiCa2C3H (s) + 5NH4Cl (s) → 2CaCl2 (s) + LiCl (s) + 5NH3 (g) + C3H4 (g) + H2 (g)

The mass spectra of aliquots of gaseous products feature a strong signal at 39 m/z, confirming production of C3H4. This could be in the form of propadiene or propyne or a mixture of these isomers

(this was seen for the protolysis of Mg2C3), although the envelope of isotopes from m/z 35-40 better matches that of propadiene.37 Both gases are of interest for industrial uses (as gas welding fuels, either pure or in a methyl acetylene-propadiene-propane mixture known as MAPP gas), and they are also convenient building blocks for organic synthesis. The reaction of elemental carbon to form LiCa2C3H

and the ready protolysis of this phase to form C3H4 may be a valuable method to convert carbon into useful multiple bonded reactants for organic syntheses or to produce isotopically enriched precursors.38

Mg2C3 is also of interest for this. However, synthesis of this compound involves reaction of Mg powder with flowing pentane gas at high temperatures, or lengthy (85 h) ball milling of the elements followed by annealing; both syntheses are somewhat inconvenient.27,38 - 4- With the incorporation and full occupancy of the H and C3 anions, the title phase has a charge- + 2+ 4- - balanced composition, Li (Ca )2 (C3) (H ). This appears to be the only reported structure with ordered coexisting carbide and hydride anions. A number of studies have been carried out on rare earth carbide hydrides such as YbC0.5H and La2C3H1.5, but these are highly disordered (and in the latter case amorphous) phases with varying amounts of interstitial carbon and hydrogen.7,8 The most comparable compound is La2C2H2, which forms from the reaction of La2C3 with hydrogen at 1070K. It is theorized - 4- to contain both interstitial H (in tetrahedral sites) and C2 units (in octahedral sites), but was characterized by powder diffraction, and actual refinement of the structure from the powder data was not possible (XRD data were dominated by La scattering; the approximate structure model is based on 29 La2C2O2). 7Li MAS NMR data were collected at room temperature for products of reactions of

Ca/Li/C/CaH2 in mmol ratios of 10:10:6:x, with x = 0, 0.25, 0.5, 0.75, and 1 mmol. All spectra in Figure 2.4 exhibit the same three lithium resonances: two in the Knight-shifted metallic lithium region (286 and 270 ppm) and one in the ionic lithium region (4 ppm, referenced to LiCl). As the amount of CaH2 reactant increases, the intensity of the ionic lithium peak in the product spectrum increases. This is in accordance with the increasing yield of LiCa2C3H, indicating that this peak at 4 ppm corresponds to the lithium in this phase. The two metallic lithium peaks at 286 and 270 ppm always occur at a 3:1 ratio.

They are likely due to CaLi2; this intermetallic will be formed in the solidified residual traces of Ca/Li flux adhering to the solid products after centrifugation. Weak peaks in the powder XRD patterns for these samples indicate the presence of this phase. CaLi2 has the C14 Laves phase structure (hexagonal 39 space group P63/mmc), with lithium atoms in 2a and 6h Wyckoff sites. The two lithium sites are in the proper ratio to consistently produce NMR peaks in a 3:1 ratio, and their Knight shifted resonances (close to the 290 ppm for elemental lithium) are in agreement with the metallic nature of this compound.

x = 1.0 * *

x = 0.75

x = 0.50

x = 0.25

x = 0

400 300 200 100 0 -100 ppm

7 Figure 2.4. Li MAS NMR spectra of LiCa2C3H samples synthesized using Ca/Li/C/CaH2 mmol ratios of 10:10:6:x. Spinning sidebands are marked with asterisks.

13 The C MAS NMR spectrum of LiCa2C3H (Figure 2.5) exhibits a broad asymmetric peak, which can be deconvoluted into a large peak at 126 ppm and a smaller peak at 225 ppm, in a roughly 2:1 ratio. This is in agreement with the two crystallographic carbon sites and their associated multiplicity (terminal carbon atoms on 4g sites and the central carbon atom on a 2d site). Molecular allene exhibits resonances at 74.8 ppm (terminal carbons) and 213.5 ppm (central carbon).40 The carbon 4- 13 resonances of the anionic C3 are expected to be shifted downfield; similar shifting is seen for the C resonance of metal acetylides when compared to that of acetylene.41

13 Figure 2.5. C MAS-NMR spectrum. Collected on LiCa2C3H at 25°C in a 4 mm rotor and a 4 kHz spin rate with a pulse width of 4 µs. A recycle delay of 300 s and 800 pulses total were used.

1 LiCa2C3H exhibits only one very broad peak in its H MAS NMR spectrum, at approximately 3 ppm (Figure 2.6). The majority of reported 1H shifts for alkaline earth hydrides are more negative; shifts 42 of -4.5, -6.7, and -8.7 ppm are seen for CaH2, SrH2, and BaH2, respectively. Complex hydrides with an H- anion surrounded by alkaline earth cations also commonly exhibit negative 1H shifts, as seen for 43,44 1 Ba5Sb3H (-8.5 ppm) and Ba9In4H (-9.0 ppm). The H shift of +3 ppm indicates that the hydrogen nuclei of LiCa2C3H are deshielded as compared to typical metal hydrides, possibly due to polarization + of the electrons by the neighboring Li cations. It is notable that LiH, MgH2, and AlH3, phases with considerable polarization and covalence in their metal-hydrogen interactions, also have 1H resonances around 3 ppm.36

2.86 ppm

1 Figure 2.6. H MAS-NMR spectrum of LiCa2C3H. Collected at 25°C in a 4 mm rotor and a 8 kHz spin rate with a pulse width of 5 µs. A recycle delay of 1 s and 16 pulses total were used.

Electronic Calculations. The density of states (DOS) diagram for LiCa2C3H is shown in Figure 2.7. In agreement with its charge balanced stoichiometry, the compound is a semiconductor with a small band gap (Eg = 0.48 eV). Because of the air sensitivity of this phase, diffuse reflectance spectroscopy and resistivity measurements could not be carried out to confirm the band gap. However, the silver color and reflectance indicates the band gap is below 1 eV. The hydride bands are narrow and largely localized at around -4 eV below the Fermi level; the lithium derived states are also found in this energy 32 range. Similar localization and hybridization between Li and H states was reported for Li4Si2H. Both

phases also feature short Li-H bonds as compared to the sum of their ionic radii; strong Li-H interactions may act to stabilize these phases. The states just below Ef are dominated by contributions from carbon p-orbitals and calcium d-orbitals.

Figure 2.7. Total and partial density of states for LiCa2C3H. Contributions from Ca, Li, C and H are color-coded.

CHAPTER 3

LiCa7Si3H3

3.1 Introduction

Given the unusual carbide hydride (LiCa2C3H) that was produced from reaction of carbon and

CaH2 in Ca/Li flux, exploration of reactions of tetrelides and hydrides in Ca/Li melts was continued by replacing the carbon reactant with silicon. LiCa7Si3H3 is produced from these reactions. This phase exhibits a new structure type that forms in the orthorhombic Pnma space group with unit cell parameters of a = 9.8371(36) Å, b = 13.7352(50) Å, c = 8.7191(32) Å. The structure incorporates the Si4- anion and three different clearly defined hydride anions. Si normally forms covalently bound 45,46 clusters as seen in Ca2LiSi3 and other similar structures. Other calcium silicide hydride phases have 47 been observed such as CaSiHx but do not have well-defined crystallographic sites. The electronic properties of LiCa7Si3H3 were explored using density of states calculations and NMR spectroscopy studies; both identify the phase as a small-gap semiconductor. The 29Si resonances are similar to those observed in prior NMR work done on the chemically comparable CaMgSi phase.47

3.2 Experimental procedure

Silicon (Alfa Aesar, 99.9%) and CaH2 (98%, Alfa Aesar) were combined in a flux comprised of equimolar ratios of calcium shot (99.5% Alfa Aesar) and lithium rod (99.8% Strem Chemicals). The

Ca/Li/Si/CaH2 ratios explored were 10:10:1:1 and 10:10:3:2. Reactants were loaded into either stainless steel crucibles or niobium crucibles which were welded shut under argon and then sealed into fused silica tubes under vacuum. All reaction ampoules were heated to 1323 K in 2 h and were kept at this temperature for 2 h. They were cooled to 1073 K in 24 h, and then cooled to 773 K in 108 h. At 773 K, the tubes were removed from the furnace, inverted, and centrifuged to force the excess molten Ca/Li mixture off of the product crystals. Products were stored and handled in an argon atmosphere glovebox to prevent degradation. Elemental analysis was carried out using SEM-EDS. Crystals were affixed to an aluminum SEM stub using carbon tape and analyzed using a 30 kV accelerating voltage and an accumulation time of 60 s. For single crystal X-ray diffraction studies, sample crystals were brought out of the glovebox under Paratone oil. Small spheroid pieces cleaved from the larger bulk crystals were mounted in a cryoloop. Single crystal X-ray diffraction data were collected at 150 K in a stream of nitrogen using a

Bruker APEX 2 CCD diffractometer with a Mo Kα radiation source. Crystallographic data are listed in Tables 3.1, 3.2, and 3.3.

Table 3.1 Crystallographic data collection parameters for LiCa7Si3H3

LiCa7Si3H3 Formula weight (g/mol) 374.7 Crystal system orthorhombic Space group Pnma Z 4 Unit cell parameters (Å) a = 9.8371(36) b = 13.7352(50) c = 8.7191(32) Volume (Å3) 1178.08 Calculated density (g/cm3) 2.360 Absorption coefficient (mm-1) 2.18 Theta range 8.80<θ<45.91 Data / restraints / parameters 9718/0/64 GoF on F2 1.053 R1, wR2 [I>2σ(I)] 0.0321, 0.0754 R1, wR2 (all data) 0.0477, 0.0754 Max., min. resid. (e·Å-3) 0.47, -1.13

Table 3.2 Atomic position and thermal displacement parameters for LiCa7Si3H3

[b] Atom Wyckoff x y z Ueq site

Ca(1) 8d 0.08983(6) 0.10917(9) 0.0546(6) 0.0119(9) Ca(2) 8d 0.11295(3) 0.61021(3) 0.32924(8) 0.01174(6) Ca(3) 8d 0.18576(2) 0.11871(4) 0.45436(3) 0.0121(2) Ca(4) 4c 0.38432(6) ¼ 0.19677(6) 0.0117(5) Li(1) 4c 0.13483(3) ¼ 0.75559(6) 0.0172(3) Si(1) 8d 0.36860(2) 0.02501(6) 0.21227(6) 0.0131(3) Si(2) 4c 0.3935(2) ¼ 0.5747(2) 0.0124(5) H(1) 4c 0.0282(9) ¼ 0.5400(8) [c] H(2) 4c 0.132(4) ¼ 0.253(1) [c] H(3) 4c 0.7376(9) ¼ 0.531(1) [c] [c] Hydrogen atoms refined isotropically.

Table 3.3 Bond lengths observed for LiCa7Si3H3. Bonds in Length (Å)

LiCa7Si3H3 Si(1) – Ca 3.194(1)-3.543(2) Si(2) – Ca 2.935(1)-2.957(2) Si(2) – Li(1) 3.225(1)

H(1) – Li(1) 2.12(1) H(1) – Ca 2.45(3)-2.70(6) H(2) – Ca 2.47(5)-2.57(2) H(3) – Li(1) 2.15(3) H(3) – Ca 2.49(4)-2.62(8)

Magic angle spinning (MAS) 7Li, 1H, and 29Si NMR data were collected on a Varian Inova 500 wide-bore spectrometer at room temperature. LiCl powder was used as external reference for 7Li and 1 29 tetramethylsilane (TMS) for H and Si. In a dry box, crystals of LiCa7Si3H3 grown from a flux reaction ratio of 10:10:3:2 were ground; the mixtures were packed into 4.0 mm zirconia rotors with airtight screw caps. 7Li NMR data were collected at 25 °C and at 15 kHz spin rate. A one-pulse sequence was used with a pulse width of 5 µs and a recycle delay of 15 µs. A total of 400 scans were collected for each spectrum. The 29Si spectrum was collected on a sample in a 4 mm rotor spinning at 5 kHz with a pulse width of 5 us and a recycle delay of 10 s; 20000 pulses were totaled. The 1H spectrum was collected for a sample in a 4 mm rotor spinning at 8 kHz. The 1H spectrum is the sum of 1600 pulses, each with a pulse width of 5 µs and a recycle delay of 1 s.

3.3 Results and discussion

LiCa7Si3H3 grows in a 10:10 mmol ratio Ca/Li melt as silvery, reflective, rod-like crystals. The larger individual crystals are up to 1 mm in length, shown in an electron micrograph (Figure 3.1). These single crystals appear multifaceted and form into rods. This phase only appears in powder X-ray diffraction patterns when CaH2 is added to the flux. Powder patterns also reveal a few peaks associated with silicide side products. In particular, Ca3Ni3Si2 crystals are found in all reactions carried out in steel crucibles. The silicide-rich Ca/Li melt etches nickel from the stainless steel and induces formation of this byproduct. While this phase is of interest due to its new structure type (a monoclinic distorted variant of the previously published Pnma type; see figure 3.2), attempts to minimize its formation were sought by using different crucibles. Reactions carried out in niobium ampoules did eliminate formation

of Ca3Ni3Si2, but the yield of the desired LiCa7Si3H3 was also lowered, likely due to loss of hydrogen through the niobium crucible (niobium becomes porous to hydrogen gas when heated above 800°C). Reactions in pure iron jackets within stainless steel crucibles are being investigated to solve these problems.

Figure 3.1. SEM micrograph of a crystal of LiCa7Si3H3.

C2/m Pnma

c A a B

Ca Ca

A a

Figure 3.2. Structural variants of Ca3Ni3Si2; the Ca/Li grown variant has an alternate stacking of Ni-Si layers leading to monoclinic symmetry.

Si(1)

Si(2)

Figure 3.3. Structure of LiCa7Si3H3.

4- - 2+ LiCa7Si3H3 is a charge-balanced Zintl phase comprised of isolated Si and H anions and Ca + + 2+ 4- - and Li cations: (Li )(Ca )7(Si )3(H )3. This phase crystallizes in an orthorhombic structure shown in Figure 3.3 which can be viewed as layers of Ca/Si separated by slabs containing both silicide and hydride anions as well as Li+ cations. In the Ca/Si slab, Si(1) atoms are coordinated by 10 calcium atoms at distances ranging from 3.1941-3.5434Å (see Table 3.3); the Si-Ca interatomic distances in 48,49 Ca2Si are also in this range. The Si(2) atoms in the hydride slab are coordinated more strongly to six calcium atoms in a trigonal prismatic distribution with bond lengths of 2.9351-2.9570Å. These trigonal prisms share edges with hydride-centered octahedra to form the hydride silicide slabs (see Figure 3.4).

Figure 3.4. View of hydride layer in LiCa7Si3H3 (viewed down the b-axis), and coordination environment of the three hydride sites.

Location of the hydride anions in the structure was reasonably straightforward. After assignment of the heavy atom sites, three additional peaks in the residual electron density of ~1 e Å3 became apparent in the final stages of structural refinement. All three hydride sites in the structure are octahedrally coordinated by cations. While tetrahedral coordination is more common for hydride anions due to their small size, octahedral coordination is seen in LiCa2C3H, Ba9In4H, Ca6[Cr2N6]H, and 13,23,44,50,51 AE5Pn5Hx hydrides with the stuffed Mn5Si3-type such as Sr5As3H. Ca6[Cr2N6]H and

LiCa2C3H exhibit Ca-H distances of 2.6491(4)Å and 2.5016(1)Å respectively. These are within the range of Ca-H distances seen in LiCa7Si3H3. The octahedral siting of the hydrides in LiCa7Si3H3 (and + LiCa2C3H) may be stabilized by the presence of Li in the coordination sphere. Of the three hydride sites in the structure, two of them (H(1) and H(2)) are coordinated to five Ca2+ cations and one Li+ ion. The lithium site is shared between these octahedra, resulting in a linear H—Li—H unit with Li-H distances of 2.120 Å and 2.153 Å. While these Li-H distances are longer than the 1.875Å bond length observed in LiCa2C3H, it is notable that the carbide hydride structure also features a structural motif of lithium-hydride chains (in that case, an infinite H-Li-H-Li- chain running along the c-axis), as does the

LiCa11Ge3OH4 structure (see chapter 4). 7Li, 29Si, and 1H MAS NMR spectroscopy experiments were carried out to further explore the structural and electronic properties of this phase. The lithium spectrum shows one resonance at 4 ppm and two resonances at 283 and 291 ppm (see Figure 3.5a). The 4 ppm resonance is due to the ionic lithium ions in LiCa7Si3H3, and the two Knight-shifted peaks near 300 ppm are due to traces of CaLi2 (a metallic alloy formed when Ca/Li flux is cooled) coating the sample. This is very similar to what was 7 1 observed in Li spectra of LiCa2C3H (Figure 2.4). The H MAS NMR spectrum (figure 3.5b) shows a very broad resonance centered around 4 ppm (with respect to TMS). This hydride chemical shift is similar to that of LiCa2C3H, LiH and MgH2; this may indicate some amount of covalence in the Li-H 1 interaction. More strongly ionic hydrides such as SrH2 and Ba9In4H have more negative H resonances

(-7 ppm and -9 ppm respectively) 8. Higher resolution spectra are being sought to deconvolute the three expected hydride signals. The 29Si MAS NMR spectrum (figure 3.5c) shows two sharp resonances, at 158 ppm and 230 ppm (referenced to TMS), in approximately 2:1 ratio. This ratio is in agreement with the Si(1)/Si(2) multiplicities and the peaks can therefore be assigned to these two silicon sites. The observed chemical shifts are slightly outside the normal range for charge-balanced Zintl phase silicides (-350 ppm to 50 ppm). They are less strongly shifted than metallic silicides such as Rb8Na16Si136 which exhibit Knight 52 shifts (270 to 800 ppm) due to conduction electrons. The observed silicon resonances for LiCa7Si3H3 appear most similar to the shifts seen for semiconductor/semimetal CaMgSi (160 ppm) and poor metal

Ba3Si4 (82 ppm, 281 ppm), supporting the identification of this phase as a small bandgap semiconductor.53

a) 7Li

* *

400 350 300 250 200 150 100 50 0 -50 -100

ppm b) 1H

* *

ppm

c) 29Si

* *

ppm Figure 3.5. MAS NMR spectra of LiCa7Si3H3; spinning sidebands marked by asterisk.

3.4 Thermo-volumetric Measurements

Powder samples are heated in a custom built air tight chamber filled with Ar. LiAlH4 decomposition was used as a standard to compare to similar decomposition of the new LiCa7Si3H3 compound. LiAlH4 showed two pressure increases at ~150°C and 200°C when slowly heated at 2°C/minute up to a temperature of 300°C (Figure 3.6). These peaks concur with reported decomposition 23 temperatures for this phase . Decomposition has not been determined yet for LiCa7Si3H3. Issues with purity of samples and sample size may have prevented the detection of a decomposition peak.

LiAlH4 decomposition

30 PSI 20 1st hydrogen gas forming hydride decomposition 10 nd decomposition 2 0 0 50 100 150 200 250 300 350 Temperature, degrees C

Figure 3.6. Hydride decompositions of LiAlH4 standard material are shown by the 2 spikes in pressure that are marked.

CHAPTER 4

LiCa7Ge3H3 AND LiCa11Ge3OH4

4.1 Introduction The continuation of this work using germanium instead of carbon or silicon has led to the formation of two new phases. Germanium reacts with CaH2 in Ca/Li flux to form LiCa7Ge3H3, a Zintl phase hydride isostructural with LiCa7Si3H3. In the presence of oxide contamination (or deliberate addition of CaO as a reactant) formation of LiCa11Ge3OHx (x ≈ 4) is observed. Both structures feature isolated Ge4- anions coordinated to 9-10 calcium cations, and hydride anions octahedrally coordinated 2+ + by Ca and Li ions. The LiCa11Ge3OH4 phase does not appear to be charge-balanced and has a complex structure which, in addition to Ge4- and H- anions, also features oxide anions octahedrally coordinated by calcium cations.

4.2 Experimental procedure All manipulations of air-sensitive reactants and products were carried out in an argon-filled glovebox. Reactants included calcium shot (99.5%, Alfa Aesar), lithium rod (99.8%, Strem Chemicals), calcium hydride powder (98%, Alfa Aesar), calcium oxide powder (99%, Fisher Chemicals) and germanium powder (99.9%, Acros Organics). A variety of reaction ratios were explored using both commercial calcium and distilled calcium to make the Ca/Li flux. Reactants were weighed out and placed into steel crucibles (6.5 mm ID) which were arc-welded shut under an argon atmosphere; these ampoules were placed in fused silica jackets sealed under a vacuum of 10-2 Torr.

All reaction ampoules were heated to 1323 K in 2 h and were kept at this temperature for 2 h. They were cooled to 1073 K in 24 h, and then cooled to 773 K in 108 h. At 773 K, the tubes were removed from the furnace, inverted, and centrifuged to force the excess molten Ca/Li mixture off of the product crystals. Much of the crystalline product adheres to the surface of the steel crucible during growth and centrifugation, so use of a filter to separate the crystals from the flux is not necessary. Crucibles were cut open in a glovebox under an argon atmosphere. The solid product consists of silver reflective rectangular crystals up to 1 mm long in the case of LiCa11Ge3OHx and silver multi-faceted crystals up to 1 mm long in the case of LiCa7Ge3H3. Both phases are extremely air- and water- sensitive. Elemental analysis was performed using SEM-EDS. This technique cannot analyze light atoms, but the calcium and germanium ratios confirm those determined from the structure refinements (see below). No iron was detected in the product crystals.

Small amounts of LiCa7Ge3H3 and LiCa11Ge3OHx phases were originally found from reaction of

Ge and CaH2 in a Ca/Li flux formed from commercial calcium contaminated with hydride and oxide; a large amount of binary phase Ca2Ge was also produced in this reaction. Addition of larger amounts of

CaH2 as a reactant in this flux (10:10:2:3 mmol ratio of Ca/Li/Ge/CaH2) increases the yield of

LiCa7Ge3H3 and LiCa11Ge3OHx (20% based on Ge). The highest yield of LiCa7Ge3H3 (50%) is produced using distilled calcium and deliberate addition of CaH2, with a Ca/Li/Ge/CaH2 mmol ratio of

10:10:2:3. Addition of CaO was explored to improve the yield of LiCa11Ge3OHx. Reactions with a

10:10:2:3:1 mmol ratio of Ca/Li/Ge/CaH2/CaO produced larger amounts of LiCa11Ge3OHx; LiCa7Ge3H3 was also formed in this reaction. Powder X-ray diffraction studies were carried out on reaction products to determine identities of byproducts and to explore the effects of changing reaction stoichiometry. For single crystal X-ray diffraction studies, small spheroid pieces cleaved from the larger bulk crystals were mounted in cryoloops using Paratone oil. The LiCa7Ge3H3 structure was solved in orthorhombic space group Pnma; details of the refinement can be found in tables 4.1 and 4.2. The calcium and germanium sites were found by direct methods and the lithium and hydrogen sites by analysis of the difference Fourier map and consideration of bondlengths. Anisotropic refinement of thermal parameters was possible for all atoms except for the hydrogens; these were refined isotropically. During the final refinement cycles occupancies of all atoms were allowed to vary and were not found to vary significantly from unity. The

LiCa11Ge3OH4 structure was solved in tetragonal space group P42/nmc (see table 4.1 and 4.3). Location of Ca, Ge, and O sites were straightforward. Three data sets were compared to determine consistant hydride sites. Refinement of hydride occupancies were not possible.

Calculations of density of states (DOS) for LiCa7Ge3H3 were performed with the tight binding- linear muffin tin orbitals-atomic sphere approximation TB-LMTO-ASA program package, based on the 25 experimentally determined unit cell dimensions and atomic coordinates for LiCa7Ge3H3. The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. The calculation was made for 1056 κ points in the irreducible Brillouin zone. Integration over the Brillouin zone was made by the tetrahedron method.25 The following radii of atomic spheres were applied for the calculations: r(Ca) = 3.78 Å, r(Li) = 2.74 Å, r(H) = 1.35 Å, r(Ge) =2.34 Å. The basis sets contained Ca(3s, 3d), Li(2s), H(1s) , and Ge(4s, 4p) with Ca(3p), Li(2p, 3d), H(2p, 3d), and Ge(4d) functions being downfolded.

Table 4.1. Selected crystal data and structural refinement parameters for LiCa7Ge3H3 and LiCa11Ge3OH4.

LiCa7Ge3H3 LiCa11Ge3OH4 Formula weight (g/mol) 508.3 695.0 Crystal system orthorhombic tetragonal Space group Pnma P42/nmc Z 4 8 Unit cell parameters (Å) a = 9.8599(2) a = 15.832(2) b = 13.7713(3) c = 13.694(2) c = 8.7430(2) Volume (Å3) 1187.18(4) 3432.3(8) Calculated density 2.844 2.690 (g/cm3) Absorption coefficient 10.504 8.552 (mm-1) Theta range 2.76º to 34.93º 1.82º to 25.35º Data / restraints / 2682 / 0 / 65 1704 / 0 / 99 parameters GoF on F2 1.035 1.052 R1, wR2 [I>2σ(I)] 0.0195, 0.0405 0.0350, 0.0717 R1, wR2 (all data) 0.0248, 0.0421 0.0537, 0.0785 Max., min. resid. (e·Å-3) 0.820, -0.794 0.987, -1.072

[a] Table 4.2. Atomic coordinates and thermal displacement parameters for LiCa7Ge3H3.

[b] Atom Wyckoff x y z Ueq site Ca(1) 8d 0.08986(3) 0.10915(2) 0.05499(3) 0.00771(6) Ca(2) 8d 0.11268(3) 0.61025(2) 0.32934(3) 0.00724(6) Ca(3) 8d 0.18597(3) 0.11888(2) 0.45436(3) 0.00661(6) Ca(4) 4c 0.38456(4) ¼ 0.19600(4) 0.00655(7) Li(1) 4c 0.1354(4) ¼ 0.7553(5) 0.0147(8) Ge(1) 8d 0.36828(2) 0.02498(1) 0.21149(2) 0.00740(4) Ge(2) 4c 0.39418(2) ¼ 0.57552(2) 0.00602(5) H(1) 4c 0.031(4) ¼ 0.544(4) [c] H(2) 4c 0.128(3) ¼ 0.251(3) [c] H(3) 4c 0.745(4) ¼ 0.534(4) [c] [a] All site occupancies are 100%. [b] Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. [c] Hydrogen atoms refined isotropically.

[a] Table 4.3. Atomic coordinates and thermal displacement parameters for LiCa11Ge3OH4.

[b] Atom Wyck x y z Ueq off site Ca(1) 16h 0.05905(8) 0.11229(8) 0.09624(9) 0.0098(2) Ca(2) 16h 0.07144(8) 0.02688(8) 0.35106(9) 0.0103(3) Ca(3) 16h 0.63592(8) 0.06823(3) 0.0014(1) 0.0096(2) Ca(4) 8g ¼ 0.0223(1) 0.2040(1) 0.0099(4) Ca(5) 8g ¼ 0.0504(1) 0.7960(1) 0.0212(5) Ca(6) 8g ¼ 0.1491(1) 0.4104(1) 0.0207(4) Ca(7) 8g ¼ 0.6422(1) 0.6464(1) 0.0091(4) Ca(8) 4d ¼ ¼ 0.1564(1) 0.0122(5) Ca(9) 4c ¾ ¼ 0.5835(1) 0.0096(5) Li(10)/ 8f 0.3507(3) 0.6493(3) ¼ 0.023(2) Ca(10)[a] Ge(1) 8g ¼ 0.10010(6) 0.00323(7) 0.0098(2) Ge(2) 8g ¼ 0.54976(6) 0.42428(7) 0.0101(2) Ge(3) 8f 0.58886(4) 0.41114(4) ¼ 0.0091(2) O(1) 8e 0 0 0 0.025(1) H(1) 2a ¼ ¾ ¼ [c] H(2) 16h 0.388(2) 0.518(2) 0.181(2) [c] H(3) 16h 0.157(3) 0.136(3) 0.260(4) [c]

[a] All site occupancies are 100% except for Ca(10)/Li(10) (17.4(8)% Ca, 82.6(8)% Li). [b] Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. [c] Hydrogen atoms refined isotropically

4.3 Results and discussion Synthetic explorations in Ca/Li flux indicate that this reaction medium is an excellent solvent for a wide range of metals and metalloids. It also dissolves ionic compounds such as Ca3N2 and CaH2. To explore the effects of oxide and hydride impurities, reactions of germanium were carried out in Ca/Li flux under a variety of conditions: using commercial Ca to form the flux; using distilled Ca to form the flux; and with or without deliberate addition of CaH2 or CaO as reactants. Because of the high reactivity and volatility of both flux components, crucible selection is limited to sealed metal ampoules. Steel ampoules are stable to the flux, and were used in this work. One drawback of steel is the possibility of leaching iron, carbon and other alloying elements from the ampoule walls; this is particularly problematic if magnetic measurements are planned on the products. While no iron was indicated by SEM-EDS measurements, incorporation of even trace amounts of iron will affect magnetic data. Niobium or tantalum ampoules are recommended for avoiding such contamination; however, at elevated temperatures, these ampoules become permeable to hydrogen. This property has been used to remove hydride impurities from metals.48 Since synthesis in Ca/Li flux favors the formation of calcium-rich phases which incorporate a range of interstitial elements, it is extremely difficult to control the synthesis to isolate only one of the observed calcium- and germanium-containing products (Ca2Ge, LiCa7Ge3H3, and LiCa11Ge3OH4). Pure

Ca2Ge can be obtained in well-crystallized form from reaction of Ge in Ca/Li flux made with distilled Ca; the unit cell parameters of these crystals match those reported in the literature for the orthorhombic 49 structure. Similar reactions in commercial Ca/Li flux leads predominantly to Ca2Ge, but small amounts of LiCa7Ge3H3 and LiCa11Ge3OH4 are also present. The yield of LiCa7Ge3H3 is greatly increased by using distilled Ca/Li and Ge and CaH2 as reactants. This supplies a large excess of hydride

(lowering the amount of binary Ca2Ge formed) and eliminates oxide contaminants which can lead to the formation of LiCa11Ge3OH4. Similar reactions of Ge and CaH2 carried out in commercial Ca/Li flux also lead to increased amounts of the LiCa7Ge3H3 phase, but the LiCa11Ge3OH4 phase is also produced due to the oxide present in the commercial calcium. Reactions of CaH2, CaO, and Ge in the Ca/Li flux yields a mixture of several phases, with LiCa11Ge3OH4 being predominant but LiCa7Ge3H3 and Ca2Ge also present along with other unknown phases. Recrystallized CaH2 becomes prevalent when large amounts of this reactant are present in the flux (see Figure 4.1). These results underscore the importance of using distilled calcium, but also indicate the usefulness of exploring the deliberate addition of interstitial elements for materials discovery.

LiCa11Ge3OH4

LiCa7Ge3H3

25 30 35 40 45 000 10:10:1:1, Nb tube

000 10:10:2:3:1

000 10:10:2:3, distilled Ca

000 10:10:2:3

000 Ca Ge 2

CaH2

25 27 29 31 33 35 37 39 41 43 45 2Theta Figure 4.1. Powder X-ray diffraction of Ca/Li/Ge/CaH2/CaO reactions. Data for products for reactions at various listed reactant ratios, compared to calculated patterns for several expected products.

Table 4.4. Comparison of selected interatomic distances in LiCa7Ge3H3 and LiCa11Ge3OH4.

Bonds in Length (Å) Bonds in Length (Å)

LiCa7Ge3H3 LiCa11Ge3OH4 Ge(1) – Ca 3.0439(3)– 3.6861(3) Ge(1) – Ca 2.945(2) – 3.286(1) Ge(2) – Ca 2.9321(3)– 2.9639(3) Ge(2) – Ca 3.154(2) – 3.376(2) Ge(2) – Li(1) 2.800(4), 2.997(4) Ge(3) – Ca 3.039(1) – 3.500(1)

H(1) – Li(1) 2.11(3) H(1) – Li(10) 2.255(5) H(1) – Ca 2.49(2) – 2.63(2) H(1) – Ca(9) 2.280(2) H(2) – Ca 2.44(2) – 2.62(2) H(2) – Ca 2.30(3) – 2.64(4) H(3) – Li(1) 2.14(3) H(3) – Ca 2.45(4) – 2.75(5) H(3) – Ca 2.44(3) – 2.67(2) O(1) – Ca 2.371(1) – 2.408(1)

The presence of oxide in the Ca/Li/Ge/CaH2 reaction mixture leads to formation of 4- 2- - LiCa11Ge3OH4. This phase has a tetragonal structure featuring isolated Ge , O , and H anions coordinated to Ca2+ and Li+. The presence of several different anions results in a highly complex structure; this is also demonstrated by Ba21Ge2O5H24 (which features the same set of anions) and

Sr21Si2O5C6 (which is related to the barium phase by replacing the hydride anions with carbide 8,54 anions). The LiCa11Ge3OH4 structure is shown in figures 4.2 and 4.3. In figure 4.2 the Ge-centered polyhedra are featured; in figure 4.3, the hydride-centered octahedra are highlighted. The three Ge4- anion sites are coordinated by 9–10 Ca2+ atoms with Ge-Ca interatomic distances similar to those in

LiCa7Ge3H3 (see table 4.4). The oxide anions are octahedrally coordinated by calcium ions, with the small range of Ca-O distances only slightly longer than the 2.37Å observed in the inverse perovskite 55 Ca3GeO.

4- Figure 4.2. The LiCa11Ge3OH4 structure, viewed down the c-axis. Ge coordination spheres represented as grey polyhedra and oxide-centered octahedra in red. Calcium, lithium, and hydrogen atoms are blue, green, and yellow, respectively.

The stoichiometry of LiCa11Ge3OH4 is not charge-balanced; it is possible that more hydride is incorporated in the compound but not found in the structural refinement. While three single crystal X- ray diffraction datasets of this phase were analyzed and compared to locate consistent hydride sites (see Experimental Section), it is likely that some have been missed, particularly if the hydride ions are distributed over several partially occupied sites. Of the samples that were analyzed by powder and single crystal diffraction, the unit cells did not vary significantly with reactant ratio, which may indicate that this compound is a Zintl phase with no significant phase width. Synthesis of a fluoride analog - (LiCa11Ge3OFx) would aid in the structural analysis; F can in many cases occupy hydride sites, and this heavier atom is easier to locate in a crystal structure refinement. However, reactions of Ge, CaO, and

CaF2 in Ca/Li flux have not yielded isostructural products.

a) b)

Ge Ca

H Li

c) Li(10)

H(1) 2.255Å

2.36Å H(2)

Figure 4.3. The LiCa11Ge3OH4 structure, highlighting the locations and connectivity of the hydride- centered octahedra (yellow). a) Structure viewed down the c-axis. b) Structure viewed down the a- axis. c) Lithium-hydride cluster.

Three hydride sites were definitively located in the structure. All three are octahedrally coordinated, with a wider range of Ca-H and Li-H distances than is seen for the charge-balanced

LiCa7Ge3H3. In particular, two shorter Ca-H bond lengths are observed (2.280(2) and 2.30(3)Å).

However, these bond lengths compare well to the 2.30 – 2.36Å range seen in CaH2, and to the sum of the ionic radii for these elements.13,31 The hydride-centered octahedra share edges and faces to form hydride layers in the ab-plane (see Figure 4.3b). It is notable that the mixed Li(10)/Ca(10) site is adjacent to two of these hydride sites, resulting in formation of a H-Li-H-Li-H cluster in the structure (figure 4.3c). The Li-H bondlengths are longer than expected, likely due to the mixing of Ca on the Li site.

LiCa7Ge3H3 is isostructural to LiCa7Si3H3 (see Chapter 3 and Figure 3.2). As a charge-balanced

Zintl phase, LiCa7Ge3H3 is expected to be a semiconductor. Band structure calculations (LMTO) indicate that this compound has a small bandgap (Eg = 0.5 eV). Bands at the Fermi level are comprised predominantly of Ge states; Li and H states are found well below Ef and appear to be interacting strongly. Similar Li/H interactions were seen for LiCa2C3H. Band structure calculations were also performed on the the isostructural LiCa7Si3H3 phase. DOS are similar between LiCa7Ge3H3 and

LiCa7Si3H3 aside from the silicide analog having a slightly larger band gap.

140 Total Ca 120 Ge 100 Li H 80

(states/eVDOS cell) 20

0 -8 -6 -4 -2 0 2 4 6 8 Energy (eV)

Figure 4.4. Density of states diagram for LiCa7Ge3H3. Partial density of states contributions from each element are color coded.

4.4 Future work Hydride contamination in calcium and other alkaline earths is often viewed as problematic, but deliberate addition of CaH2 in reactions can lead to discovery of new complex hydrides. Reactions of germanium with CaH2 and CaO in Ca/Li flux produce LiCa7Ge3H3 and LiCa11Ge3OH4; both structures feature isolated Ge4- and H- anions. Another structural feature these phases share is the clustering of lithium and hydride sites. Lithium lowers the melting point of the flux, and it may also promote the formation of Li-H rich regions in the structures growing from the melt. Additional hydride sites are likely to be present in LiCa11Ge3OH4 to balance the charge of the metal cations; synthesis of deuterium or fluoride analogs is being attempted to clarify this issue.

CHAPTER 5

CONCLUSIONS AND FUTURE WORK

Synthesis in Ca/Li flux has proven to be a useful technique for the discovery of new Zintl phases and polar intermetallics. In addition to dissolving many metals and refractory elements such as carbon and silicon, Ca/Li flux also solvates ionic compounds such as CaH2, CaO, Ca3N2, and LiF. In particular, dissolution of CaH2 makes Ca/Li flux a very promising medium for growth of new metal hydride phases, which are of great recent interest as hydrogen storage materials. In addition to hydride 4- formation, products have been isolated containing the rare C3 carbide anion, which features C-C double bonds and can be viewed as deprotonated allene. Most metal carbides feature monoatomic 4- 2- 4- carbide anions (C ), with acetylide (C2 ) or ethenide (C2 ) anions being less common and more 4- complex carbide anions such as C3 rarer still. Several products containing this unusual anion have been produced from reactions of carbon in Ca/Li flux (in addition to the LiCa2C3H phase described in

Chapter 2, the Ca12S(C3)2(CxH1-x)4 compound described below and the Ca11Sn3C8 phase recently 4- synthesized by another group member both contain the C3 anion). This indicates that this flux chemistry may be a source of even larger and more complex carbide anions. Further reactions of hydride, carbon, and other elements (such as B, P, S, Zn) in Ca/Li should be explored. Some initial promising results of reactions with boron or sulfur which still await optimization are briefly described below. Another reaction parameter to investigate is the heating profile. All reactions carried out in this work have used a fairly high maximum temperature (1050°C). While high temperatures are needed in some cases to facilitate dissolution of reactants, it may be feasible to run reactions at lower temperature with longer soak times. Lower temperatures may enable the formation of products featuring larger anionic clusters. Another modification to be investigated is changing the alkaline earth metal. Like calcium, the heavier alkaline earth metals (Sr, Ba) also produce low-melting mixtures when combined with lithium (equimolar mixtures of Sr/Li melt at 240°C; Ba/Li melts at 300°C).

5.1 Ca12S(C3)2(CxH1-x)4 The reaction of sulfur and carbon in Ca/Li flux (10:10:1:2 mmol ratio of Ca/Li/S/C) produces 4- pale blue crystals of Ca12S(C3)2(CxH1-x)4, a new phase containing C3 units. Hydride is also

incorporated from the CaH2 impurity in the commercial calcium metal used in the flux. The progress on this phase is hindered by low yield and difficulties in reproducing it. In addition, the structure 4- features a large amount of disorder, including a split Ca site, disorder in the C3 orientation, and mixing of C4- and H- anions on one of the sites. Accurate refinement of the C/H occupancy on the latter site was not possible, but if the compound is assumed to be a Zintl phase, an 83% C4-/ 17% H- ratio would be required for charge balancing. The cubic structure (Im-3 space group, a = 9.475(1)Å, R1 = 0.051) and building blocks are shown in figure 5.1. The blue coloration of this compound is particularly puzzling. Most phases grown from Ca/Li flux are small band gap semiconductors with the corresponding black or silver color. A pale blue color may correspond to a wide band gap -. semiconductor (Eg > 3.0 eV) doped with sulfur radicals (such as S3 ) as is the case for the sulfur- containing mineral sodalite (blue variation lapis lazuli).

a) b)

1.325 Å

c) calcium sulfur 2.371 Å carbon 2.471 Å

Figure 5.1. Structure of Ca12S(C3)2(CxH1-x)4. a) The full structure, highlighting the coordination of 4- sulfur by 12 Ca ions. Ca split site not shown in this figure. b) C3 anion coordination is similar to Ca2LiC3H. Split position of the terminal carbons (on symmetry equivalent sites) may be influenced by splitting of the Ca site. c) Isolated monatomic anion in an octahedral site surrounded by Ca cations. For charge balance, this site should be occupied by 83% C4- and 17% H-.

5.2 (Ca/Li)15(CBC)6(C2)2(C)x The reaction of boron and carbon in Ca/Li flux made from distilled calcium metal (Ca/Li/C/B mmol ratio of 10/10/2/1) produces a high yield of (Ca/Li)15(CBC)6(C2)2(C)x, a new boron carbide phase featuring the CBC5- anion. The structure forms in the cubic space group Ia-3d with a unit cell parameter a = 16.546(1)Å. The formation of this phase requires the use of distilled calcium; if normal (contaminated) calcium is used to create the flux, the predominant products are oxide and/or hydride phases such as LiCa2C3H. While the structure refinement appears to be well-behaved (R1 = 0.013), the resulting stoichiometry is not charge balanced and the structure contains considerable disorder, including extensive Ca/Li site mixing and large thermal parameters on the C2 unit. 5- 4- The (C=B=C) anion is isoelectronic with the C3 anion. The distinction between boron and carbon is difficult in an X-ray structure refinement due to their similar electron counts. However, the observed bondlengths indicate that the central atom in this 3 member chain is boron. The B=C 4- distances are 1.41Å, longer than the 1.32Å seen for the C=C bond in C3 , but shorter than single B-C bonds observed in other compounds. The coordination to surrounding Ca2+ cations also point to the central atom being boron. The B-Ca bonds range from 2.75 – 2.86Å, longer than the C-Ca bonds in this and other phases (2.51 – 2.70Å). The CBC5- anion is observed in other phases in the literature, such as 12 Ca5Cl3(C2)(CBC).

Another similarity to Ca5Cl3(C2)(CBC) is the presence of acetylide anions which exhibit disorder. The observed bond length in the acetylide unit of (Ca/Li)15(CBC)6(C2)2(C)x is 1.05Å, which is far shorter than the expected 1.2Å for an average carbon-carbon triple bond. However, large thermal parameters were observed for these carbon atoms in the structure refinement. This may indicate the presence of a split site, meaning the actual carbon-carbon distance is longer. Raman measurements to observe the C-C stretching modes might shed light on this. Similar behavior was reported for the acetylide anions in Ca5Cl3(C2)(CBC), which had an averaged C-C bondlength of (1.06 Å) but large thermal parameters indicating site splitting or statistical displacement.

Carbon Calcium Boron

b) c) d)

Figure 5.2. Structure of (Ca/Li)15(CBC)6(C2)2(C)x. a) Full unit cell. b) Thermal ellipsoids and 2- 5- environment of C2 units. c) Environment around CBC chain. d) Polyhedron depiction of environment around isolated C4-.

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BIOGRAPHICAL SKETCH

Education University of Central Florida, in Chemistry, 2004 - Dec, 07. Florida State University, 2007-present. Research Experience Researched projects involving growth of hydrogen storage alloys, magnetic materials, semiconductors, and ionic compounds from metal flux. I preformed synthesis of carbon C3 allene units from flux-grown ionic compounds. I made hydrogen storage measurements on intermetallic and polymer complex materials. I frequently used synthetic methods that include flux growth techniques, metal purification by distillation, glove-box work, and handling of air-sensitive materials. Characterized products by Single crystal and powder X-ray diffraction, solid state NMR, elemental analysis by EDS, scanning electron microscopy, hydrogen storage measurements, DOS calculations, and mass spec. I frequently used PowerPoint and Excel to present research results.