Indian Journal of Pure & Applied Physics Vol. 45, October 2007, pp. 790-794

High Mössbauer

Usha Chandra High Pressure Physics Lab., Department of Physics, University of Rajasthan, Jaipur 302004 Email: [email protected] Received 15 February 2007; revised 17 July 2007; accepted 9 August 2007

Pressure induces myriad changes in materials by bringing order/ disorder, by changing electronic hybridization and by inducing chemical reactions. High-pressure Mössbauer spectroscopy using Anvil Cell, a well recognized tool to investigate various types of transitions in materials, has been studied. Keywords : High-pressure Mössbauer spectroscopy, Phase transition, Diamond anvil cell (DAC), Mössbauer parameters IPC Code : G01J3/28

1 Introduction 1.1 History of High Pressure Studies of materials properties at extreme pressure- P W Bridgman ( who got the noble prize in 1946 for conditions have a major impact on his contribution towards high-pressure work) was the problems in physics, chemistry, geo-science, first person to study the materials under pressure. His planetary science and materials science1-3. The effect most important contribution was the invention of of pressure on materials brings about changes in the special seal and use of sintered carbide in physical properties due to the lattice compression and place of hardened stainless . However, it was the electronic structural changes. The decrease in the Drickamer who started the study of materials under inter-atomic distances or increase in the density leads pressure using UV-Vis spectroscopy and Mössbauer 4 to metal-insulator transition , inter band electron and spectroscopy. The major breakthrough in the high- 5,6 valence transition , change in topology of the Fermi pressure field came with the use of in 1950, 7 surface and so forth. As a consequence their physical invention of diamond anvil cell in 1958 as well as properties such as electronic specific heat, super- usage of gasket in 1964. Drickamer 9 presented conductivity and magnetism undergo changes. The variation in Mössbauer parameters (IS, QS and advancement in high-pressure techniques has enabled magnetic field) on Fe, Mo, Ni, Co, Nb, Ti, V, Cu, Pd investigations of matter at pressure exceeding 500 as well as CoCl with pressure and predicted “the GPa using diamond anvil cell (DAC) and 2 development of this new tool opens up a wide variety radiation sources 8. of possible experiments including the change in Mössbauer spectroscopy is an important tool to provide information about the pressure dependence of magnetic field with density near Curie point or Nell the magnetic ordering, local symmetry and the point of alloys and compounds, the effect of distortion electronic structure of the materials through its of the field on electronic energy levels and parameters like isomer shift (IS), quadrupole splitting changes in covalency. Extension in technique to (QS) and hyperfine field (HF) Moreover these permit working with a moving source outside the cell parameters can be determined with high accuracy and allow measurements at liquid making it possible to measure relatively smaller opens up the possibility of studies of tin changes in these properties. Discontinuities in these compounds and alloys, of rare and of iodine parameters can occur at phase transitions. Beside compounds”. these advantages high-pressure Mössbauer measure- With the usage of synchrotron radiation, the study ments are not very common due to the difficulty in of materials under very high is possible. the generation of the high pressure, containment of Very high intensity photon flux, a small beam size sample in the high pressure region and determination and directionality of the beam are very suitable for the of pressure. Very few places in the world are pursuing high-pressure study with DAC even with a small the work in this field. amount of the specimen. CHANDRA: HIGH PRESSURE MÖSSBAUER SPECTROSCOPY 791

2 Production of High Pressure common axis is very crucial and essential The hard materials e.g. sapphire, for the long life of the DAC. This is done and diamond are used for pressurizing the materials. under x40 zoom by removing Diamond being transparent to UV, visible, X-ray as the interference by the wedge like structure well as γ-regions and also can be obtained with high between the diamonds. purity, is used in spectroscopy. Detailed description of (ii) Gasketing — The introduction of gasket the DAC can be seen in the review paper by brought revolution in the high-pressure Jayaraman 8. experimentation. It not only relieves some of the strains arising from nearly perfect 2.1 Experimental arrangement alignment but also acts as a supporting ring The experimental arrangement has been reported in preventing the failure of the anvils due to the 10 detail elsewhere . Each component is discussed as concentration of stresses at the edges of the follows: anvil. It also defines the sample space in which hydrostatic pressures may be (a) Mössbauer source — The absorption in the expected. For Mössbauer measurements, it Mössbauer γ-rays by the diamonds can be serves an additional purpose. It increases the appreciable especially for 14.4 keV transition of effective collimation of the Mössbauer 57 Fe (Fig. 1, Ref.11). For a typical 0.3 radiation. diamond of thickness about 2mm, the trans- mission would be only 30-40%. This requires an intense point source with small active area. (b) Diamond anvil cell — For Mössbauer measure- ments, Merrill-Bassett type is best suited (Fig. 2). The sample along with is kept between two diamonds using a gasket with a small hole. The high pressure is applied by forcing the two diamond anvils together along a common axis by means of six cap head bolts working in left hand- right hand pairs. The diamonds are fixed in piston-cylinder arrangement. Recent design of the Fig. 2 — Opposed diamond anvil configuration, with a gasket for perforated diamond anvil cell indicates advantage sample confinement in a pressure; medium; the basic part of the of such DAC in Mössbauer spectroscopy in DAC. (Ref. 8) increasing the count efficiency over the conventional one (Fig. 3, Ref.11). (i) Alignment — The alignment of the one diamond on top of the other along a

Fig. 3 — Advancement of DAC as perforated one-One can compare the Mössbauer spectrum of a 25 µm Fe foil by conventional 0.3 carat anvil set up with that of perforated one. Fig. 1 — γ-ray transmission through diamonds as a function of Note the three fold increase in the effect attributable to the large thickness for several common; Mössbauer isotopes. (Ref. 11). improvement in the signal to background of the 14.4 keV (Ref 11) 792 INDIAN J PURE & APPL PHYS, VOL 45, OCTOBER 2007

(iii) Hydrostatic pressure — To remove the transition from marcasite type orthorhombic to pyrite pressure gradient within the sample either type cubic structure along with transformation 15,16 of 2+ 2+ Ar, Xe N 2 (up to 100 GPa); He and H 2 (up to high spin Fe to low spin Fe (Fig. 4). Similar study 1Mbar) and 4:1 - mixture was carried out in stable quasi crystal Al 63.5 Cu 24 Fe 12.5 (up to 10 GPa) is used. He and Xe are found which did not show any structural transition up to to be soft even at very high-pressures and 24.2 GPa. However, Mössbauer lines exhibited increasingly used as pressure transmitters especially when samples are reactive to normally used media. (iv) Pressure determination — A chip of ruby is placed along with the sample within the hole in the gasket. Very intense R 1 and R 2 lines of ruby, excited by the laser beam of Ar or CO 2 is measured which shifts linearly with pressure 12 . (c) Detector — The transmitted γ-rays are detected using a Si PIN solid state detector having an energy resolution of about 250 eV. The electronics is sharply tuned to detect only 14.4 keV pulses. If needed the characteristics X-rays generated due to lead is removed by placing the graded shielding consisting of successive layers of lead, brass and aluminium of appropriate thickness.

3 High-Pressure Mössbauer Spectroscopy-Indian Perspectives Though high pressure studies on materials using different techniques e.g. electrical resistivity, X-ray diffraction, laser Raman spectroscopy, thermo-power measurements etc, are done in India. High-pressure Mössbauer spectroscopic studies were not done till 1999 due to various difficulties. The first ever study was carried out on α-Fe and the Mössbauer parameters (IS, QS and magnetic field) were compared with the known values by Pipkorn et al ., thus evaluating the pressure on the sample 10,13,14 inside DAC.

Among the binary compounds with FeS 2-m (marcasite) type structure, CrSb 2 is the only compound which exhibits cooperative magnetism.

Mössbauer studies on Fe xCr 1−xSb 2 (x=0.1-1.0) showed that amongst the series x=0.1 exhibits magnetic hyperfine field at iron site with a small magnetic moment at 4.2 K. Lattice parameters at room temperature reported for the series linearly decreased as one goes from chromium rich end to iron rich end Fig. 4 — High Pressure studies on (a) Room temperature but retains marcasite type orthorhombic structure. 57 Mössbauer spectra of Fe 0,03 Cr 99.97 Sb 2 system at ambient However, pressure dependent Mössbauer and 57 pressure; (b) Mössbauer spectra of Fe 0,03 Cr 99.97 Sb 2 system at 5.6 electrical resistivity studies up to 6 GPa on Gpa; (c) Normalized resistivity (with reference to resistivity at 57 57 Fe 0.03 Cr 99.97 Sb 2 system showed first order phase room pressure) at various pressures of Fe 0,03 Cr 99.97 Sb 2 system CHANDRA: HIGH PRESSURE MÖSSBAUER SPECTROSCOPY 793

Fig. 5 — Normalized resistivity (with reference to resistivity at room pressure) at various pressures of pristine La 0.8 Sr 0.2 (n 0.98 Fe 0.02 )O 3 as well as La 0.8 Sr 0.2 (n 0.8 Fe 0.2 )O 3. The top figure shows the expanded plot of the first order transition at 0.66 GPa significant broadening indicating deterioration of short range ordering with pressure 17 . High pressure electrical resistivity studies on nanocrystalline perovskites (La,Sr)(Mn,Fe) O 3 (Fig. 5) show a first order phase transition at 0.66 GPa and a subtle phase Fig. 6 — Room temperature Mössbauer spectra of transition 18 between 3.5 and 3.8 GPa. Mössbauer La 0.8 Sr 0.2 (n 0.8 Fe 0.2 )O 3 at various pressures using DAC. The velocity scale is relative to metallic Fe measurement at room pressure on the sample indicate iron to be distributed in two different environments- 4 Conclusion Fe 3+ at lower symmetry site and Fe 4+ at higher The capabilities of 57 Fe Mössbauer spectroscopy symmetry site. Pressure seems to affect the higher have been extended to extreme condition of high symmetry site transforming Fe 4+ to Fe 3+ at 0.52 GPa. pressures and cryogenic temperatures as well as high Another phase transition is seen at 3.7 GPa pressure-high temperatures, thus, permitting an representing Fe 3+ in single kind of environment 19 investigation of electronic and magnetic properties (Fig. 6). Low-temperature-high-pressure Mössbauer over a wide range of pressure (i.e. inter atomic spectrometer with cryostat (up to liquid nitrogen spacing). Attempts to interface DAC with synchrotron temperature) has also been developed and pattern of sources have ushered a new era in the field of high- 20 Fe 2O3 at 77K was reported . pressure. Because of the very high source intensity, 794 INDIAN J PURE & APPL PHYS, VOL 45, OCTOBER 2007

exposure times can be reduced by several orders of 3 Takemura K, Yusa H, Eremet M I & Chandrashekhar N V, magnitude, making continuous monitoring of the Euro J Solid State Chem, 34 (1997) 657. 4 Mcmahan A K, Physica B , 139/140(1986) 31; J Less material properties as the pressure is varied. The first Common Met, 147 (1987) 1. successful nuclear resonant scattering studies with 5 Tups H, Takemura K & Syassen K, Phys Rev Lett, 49(1982) synchrotron radiation (SR) on polycrystalline samples 177. showed that these experiments are extremely suitable 6 Jayaraman A in “ Handbook on the Physics and Chemistry of for high-pressure experiments 21 . Applying the Rare Earths” edited by K A Gschneidner (Jr) & L Eyring methods of coherent (elastic) nuclear forward (North Holland, Amsterdam) Vol 2, 1979, 575. 7 Takemura K in Science and Technology of High Pressure scattering (NFS) of SR for the investigation of Proceedings of AIRAPT 17 , edited by M H Manghnani, magnetic properties, high-pressure experiments above W J Nellis & Nicol M F, (University Press, Hyderabad) Vol 100 GPa were performed at room temperature as well 1, 2000, 440. as low temperatures. Higher pressures were reached 8 Jayaraman A, Rev Mod Phys, 55 (1983) 65; Rev Sci Instrum with the method of incoherent nuclear inelastic 57 (1986) 103. 9 Drickamer H in Physics of solids at high pressures ,edited by scattering (NIS) of SR where the phonon density of C T Tomizaka & R M Emrick (Academic Press) 1965, 313. states (DOS) of iron was measured at pressures up to 10 Chandra U, Malhotra N & Gupta A, Solid State Physics 153 GPa. High-pressure measurements applying both (India) –Proc Solid State Symp, Edited by R Mukhopadhyay, NFS and NIS have made a big progress. Studies of B K Godwal, Yusuf S M, 42 (1999) 264. extremely small samples (in the µ gm and even in sub 11 Taylor R D & Pasternak M P, Hyperfine Interaction, 53(1990) 159; Dadashev A, Pasternak M P, Rosenberg G Kh. µ gm range) are possible at pressures well above & Taylor R D, Rev Sc Instru, 72 (2001)2633. 1Mbar where other methods have no access. 12 Piermarini G J, Block S, Barnett J D & Forman R A, J Appl Another new hyperfine interaction technique—time Phys, 36 (1975) 2774. integral synchrotron Mössbauer spectroscopy has 13 Pipkorn D N, Edge C K, DeBrunner P, Pasquali G De, been developed which uses SR to measure hyperfine Drickamer H G & Fraunfelder , Phys Rev A, 135 (1964) 1604. 14 Chandra U, Siddha S, Malhotra N & Gupta A, in Advances splitting of nuclear levels in the energy domain. Time- in high pressure science and technology, edited by N Victor integral synchrotron Mössbauer spectroscopy has Jaya, M Rajagopalan & S Natarajan, (Allied Publisher, opened a new perspective especially in the study of Chennai), 2000, 257. nuclear resonances 22 . In fact, this approach (like 15 Chandra Usha in Advances in high pressure science and synchrotron Mössbauer spectroscopy) has so far only technology-Proc. II Int., Pressure Metrology workshop and Int Conf High Press Science and Technology, edited by been applied to nuclear resonances accessible also in A K Bandyopadhyay, D Varandani, Krishan Lal (NPL, conventional way. This principal has been recently Delhi), 2001, 355. considerably developed by the stroboscopic detection 16 Chandra Usha, Sharma Y K, Gupta A, Parthasarathy G & method and has already been applied to high-pressure Bandyopadhyay A K, Philos Mag Lett, 83 (2003) 273. experiments performed using 151 Eu resonance 23 in 17 Ajay Gupta,Neelima Paul, Vijayakumar V, Godwal B K, in Advances in high pressure science and technology-Proc. II EuPd 2Si 2. Int, Pressure.Metrology workshop and Int Conf High Press Science and Technology, edited by A K Bandyopadhyay, Acknowledgement D Varandani, Krishan Lal ( NPL, Delhi), 2001,351. Author is grateful to Directors, National 18 Usha Chandra & Parthasarathy G, Philos Mag Lett, 84 Geophysical Research institute, Hyderabad and (2004) 565. 19 Chandra Usha, Mudgal Prerana, Kumar Manoj, Rawat National Physical Laboratory, New Delhi,for high Rajeev, Parthasarathy G, Dilawar Nita & Bandyopadhyay A pressure resistivity and pressure measurements K, Hyperfine Interaction, 163 (2005) 129. respectively; Defence Research and Development 20 Singh Sher & Bhargava S C, Solid State Physics (India) – Organization (DRDO) and Council for Scientific and Proc. Solid State Symposium , Edited by S M Sharma, P V Industrial Research (CSIR), New Delhi for financial Sastri, Krishna, 45 (2002) 175. P S R support. Acknowledgement is due to DAAD/DTZ for 21 Wortmann G., Rupprecht K & Giefers H. Hyperfinme Interaction 144/145 (2002) 103. gifting the Mössbauer set-up. 22 L’ Abbe C, Callens R. & Odeurs J, Hyperfine Interaction, 135 (2001) 275. References 23 Serdons I, Nasu S, Callens R, Coussement R, Kawakami T, 1 Jeanloz R, Ann Rev Phys Chem , 40 (1989) 239. Ladriere J, Morimoto S, Ono T, Vyvey K, Yamada T, 2 Yoo C S, Akella J, Cynn H & Nicol M, Phys Rev B, 56 Yoda Y & Odeurs J, Phys Rev B , 70 (2004) 014109 and (1997) 140. references therein.