M. Sekar

Pressure-induced structural transition in UGa2

High-pressure X-ray diffraction has been performed on UGa2 up to 20 GPa using a diamond anvil cell. UGa2 exhibits the AlB2-type structure with space group P6/mmm at room temperature and atmospheric pressure. At about 16 GPa a reversible structural transformation to a tetragonal phase was observed. The bulk modulus of the AlB2-type phase has been determined to be ∼100 GPa, which is comparable to rare earth digallides like TmGa2 and HoGa2.

Stability of ThGa 2 in the tetragonal phase up to 62 GPa at 300 K

We have investigated the structural stability of ThGa 2 under high pressures up to 62 GPa by performing X-ray powder diffraction studies in a diamond-anvil cell. ThGa 2 exhibits a tetragonal ThSi 2 -type structure at room temperature and pressure. At about 0.2 GPa the unit-cell volume drops significantly (4%) without any change in the structure. The tetragonal structure remains stable for pressures up to as high as 62 GPa. Possible reasons for the structural stability of ThGa 2 are discussed from the view point of the influence of the number of valence band electrons in stabilizing various crystal structures.

Structural Stability and Phase Transitions in f-Electron Based Systems

The study of high pressure structural stability and equation of state of f-electron based binary intermetallics of type AXBY, where A belongs to either rare earth of actinide atom and B any other d or p block metal, is interesting from both basic as well as applied research point of view. These studies have lead to some general systematic patterns emerging. Firstly, the AB type of compounds in general stabilizes in NaCl type cubic structure and transform to CsCl type under the action of pressure. The AB2 type of compounds is very interesting and under pressure undergoes a series of structural transitions. However, the AB3 type systems are highly stable and do not show structural transitions under pressure up to as high as 30 GPa. We found that it is interesting and enlightening to explore: (i) the reason for their stability by examining the electronic structure and (ii) look for general trends in the structural transformations. In this paper, we have presented some of our studies on f-electron based intermetallics (f-IMCs), elaborate on the trends seen in the structural transitions and correlate the results obtained with the electronic structure calculations.

Cryogenic gas loading in a Mao-Bell-type diamond anvil cell for high pressure-high temperature investigations.

A simple system for loading argon fluid at cryogenic temperatures in a Mao-Bell-type diamond anvil cell (DAC) has been developed. It is done in a two step process in which the piston-cylinder assembly alone is submerged in the cryogenic chamber for trapping the liquefied inert gas. Liquid nitrogen is used for condensing the argon gas. This system is now being efficiently used for loading liquid argon in the DAC for high pressure-high temperature experiments. The success rate of trapping liquefied argon in the sample chamber is about 75%. The performance of the gas loading system is successfully tested by carrying out direct conversion of pyrolitic graphite to diamond under high pressure-high temperature using laser heated DAC facility.

Pressure induced isostructural and structural transitions in ThAl2

Abstract High pressure X-ray diffraction experiments were performed on ThAl2 up to 20 GPa. The results do not confirm the structural transition that had been reported in the literature, to have occurred at a very low pressure of 0.3 GPa. It is observed that this system undergoes an isostructural transition around 5.5 GPa and a structural transition around 12 GPa. High pressure phase is found to adopt orthorhombic structure and is stable upto 20 GPa.

High pressure electrical resistance behaviour of dialuminides of thorium and uranium

Abstract In this paper we report the pressure-induced electrical resistance behaviour of the dialuminides of thorium and uranium up to ∼8 and ∼11 GPa, respectively. ThAl 2 shows a rapid decrease in resistance up to ∼2.5 GPa and thereafter it decreases slowly. The electrical resistance of UAl 2 also decreases monotonically and it shows a collapse in resistance above a pressure of 9 GPa where our own previous high-pressure X-ray diffraction results indicated the occurrence of a structural phase transition from cubic (MgCu 2 ) to hexagonal (MgNi 2 ) phase. The marked difference in the pressure-induced electrical resistance behaviour of these two systems is discussed by taking into account the contributions from interband scattering and spin-fluctuation scattering mechanisms in ThAl 2 and UAl 2 , respectively.

Preparation and characterization of boric oxide for single crystal growth of GaP and GaAs

Boric oxide, used as an encapsulant, prevents loss of volatile components in the growth of compound semiconductors. As the material readily absorbs moisture, and as moisture content has to be kept below a certain level, preparation and handling of this material becomes an involved process. In the present paper we report the process developed for preparing boric oxide from boric acid and growth of cylindrical rods of the desired diameter. The grown boric oxide is characterized by thermal analysis. Infrared characterization is also a powerful method and the advantages of this technique as well as the problems faced in taking the IR spectrum are discussed.

High pressure structural investigation on LaGa

High pressure X-ray diffraction studies on rare-earth gallide LaGa was carried up to a pressure of ∼28 GPa using synchrotron, as well as rotating anode X-ray source in an angle dispersive mode. LaGa exhibits CrB (B33)-type orthorhombic structure (space group Cmcm) at ambient pressure. It undergoes a reversible structural phase transition from orthorhombic to tetragonal structure (Space. Group P4/mmm) at ∼5 GPa. Both the phases coexist up to the highest pressure studied. The transition to tetragonal phase has implications in predicting the new intermediate phase in the generally observed B1 to B2 transition sequence in AB type of rare-earth intermetallics. This investigation reveals that the LnX-type compounds may exhibit the structural sequence B1→ B33→ P4/mmm→ B2 under pressure. Considering the variation of lattice parameters a, b, c and also various bond lengths with pressure, the B33 to P4/mmm transition in LaGa may be displacive in nature. The zero pressure bulk modulus and its derivative for parent phase have been estimated to be B0 = 60 ± 3 GPa and  = 4.6 ± 1.5. The bulk modulus for the high pressure tetragonal phase at ∼8 GPa also has been estimated to be B8 = 77 ± 8 GPa. We have also done the non-spin-polarized total energy calculations using the full potential linearized augmented plane wave method as implemented in the WIEN2 K code which confirms the experimental results.

Structural transformation in LaGa2 under high pressure

High-pressure structural stability studies have been carried out on LaGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ∼60 GPa. A structural phase transition was initiated at a pressure of ∼12 GPa and a complete transformation to the daughter phase occurred at ∼28 GPa. The high-pressure phase was identified to be orthorhombic with lattice parameters: a = 12.79, b = 5.09 and c = 5.28 Å at 60 GPa. The bulk modulus B 0 and its derivative for the parent phase were found to be 100 ± 16 GPa and 5 ± 4, respectively. The bulk modulus Br and its derivative for the high-pressure phase were found to be 166 ± 18 GPa and 17 ± 4, respectively.

Preparation and a.c. conductivity studies of CaF2-LiF composites

Abstract A new fluoride ion conducting composite CaF 2 -LiF was synthesized by an arc-melting technique and characterized by X-ray and complex impedance analysis. A conductivity enhancement of about three orders of magnitude was obtained as a result of dispersing LiF in the CaF 2 matrix. The observed conductivity enhancement was interpreted on the basis of the space charge model for ionic conduction. Jonschers universal power law relation, σ(ω)=σ(0)+ A ω‰ was used to explain the observed frequency dispersion of conductivity. The conductivity parameters such as power law exponent n , relaxation time τ and activation energy for relaxation E τ were calculated and the results are reported.