N. R. Sanjay Kumar

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.

Twin chamber sample assembly in DAC and HPHT studies on GaN nano-particles

In this paper, we have suggested a novel idea of twin chamber sample assembly for separating ruby from the sample to overcome certain problems during high pressure?high temperature experiments using diamond anvil cell. Two holes of diameter 70?m were drilled symmetrically about the centre of the preindented area (500 ?m diameter) in a stainless steel gasket using EDM. Using ruby pressure calibration, good pressure correlation between these two holes was established up to about 15GPa. Also, high pressure ? high temperature (HPHT) experiments on the III-V compound semiconductor GaN were performed up to ~ 3GPa and 2000K using the two chamber sample assembly and a laser heated diamond anvil cell facility. NaCl, both hydrous and anhydrous, were used as the pressure transmitting media in two separate experiments. Micro-Raman spectroscopy was used to characterise the HPHT treated samples. While GaN remained stable in its wurtzite phase when heated in anhydrous NaCl medium, it transformed to Ga2O3 with rocksalt structure in the hydrous NaCl medium.

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.

Pressure induced structural phase transition in rare earth sesquioxide Tm2O3: Experiment and ab initio calculations

Among the small cation sized rare earth sesquioxides, the reported transition pressure of cubic Tm2O3 is ambiguous. Pressure induced structural phase transition in cubic Tm2O3 has been reinvestigated using the synchrotron X-ray diffraction, Raman spectroscopy, and ab initio density functional theory (DFT) calculations up to a pressure of 25 GPa. Both the X-ray diffraction and Raman spectroscopic measurements revealed an irreversible polymorphic structural phase transition from type-C cubic to type-B monoclinic at around 12 GPa, whereas the same is predicted to be 8 GPa from the density functional theory. The phase transition observed at 12 GPa is in contrast to the literature and the reasoning has been established by other studies, viz., Raman spectroscopy and DFT. A third order Birch-Murnaghan equation of state fit to the experimental compressibility curve yielded a zero pressure bulk modulus of 149(2) GPa with the pressure derivatives 4.8(5) for the parent cubic phase and 169(2) GPa with the pressure derivative 4 for the high pressure monoclinic phase, respectively. These values are in good agreement with the calculated bulk modulus of 146 and 151 GPa for the cubic and monoclinic phases, respectively. Raman modes for the monoclinic phase of Tm2O3 are measured and reported for the first time. The mode Gruneisen parameter of different Raman modes for both cubic and monoclinic phases of Tm2O3 has also been determined. The experimental results are correlated with changes in the density of states near the Fermi level, which are indicative of structural instabilities in the parent cubic structure.

Structural stability of W2B5 under high pressure

High-pressure structural stability studies have been carried out on tungsten boride W2B5 up to maximum pressure of 36 GPa using a Mao-Bell diamond-anvil cell at beamline BR-12 of the ELETTRA synchrotron facility (λ = 0.68881 Å). The hexagonal phase (S.G:P63/mmc) of W2B5 is stable up to the maximum pressure studied. The bulk modulus is estimated to be ~347 GPa using the Birch–Murnaghan equation of state. The variation of lattice parameters and bond lengths B–B and W–B have been studied and the c-axis is seen to be marginally more compressible than the a-axis.

Development of Nd-YAG laser heated diamond anvil cell facility and HPHT synthesis of WGe2

We report setting up of an Nd-YAG laser (1.06 μm) based Laser Heated Diamond Anvil Cell (LHDAC) facility to carryout High Pressure High Temperature (HPHT) experiments. Pressure is generated by squeezing sample between diamond anvils using Mao-bell type DAC and high temperature is realized by focusing an Nd-YAG laser onto the pressurized sample inside the DAC. HPHT synthesis of WGe2 has been carried out using Nd-YAG LHDAC facility.

Strutural phase transitions in rare earth sesquioxides under pressure

RE2O3 systems exist in three polymorphic forms at ambient conditions, namely: hexagonal A-type, monoclinic B-type, and cubic C-type. They show several structural phase transformations under pressure and many experimental studies have been reported in the recent times. However, the underlying mechanism which determines the stability of the cubic C-type phase for majority of the oxides under pressure and its eventual phase transformation either to the monoclinic or to the hexagonal phase has not been understood. Pressure is expected to promote the stability of A-type over C-type as the molar volume is found to decrease in the phase sequence C-B-A. In this report, the results of high pressure x-ray diffraction studies on a variety of such oxides carried out by our Group will be presented. The systematic structural transition data have been compared and contrasted with the results available for other RE2O3 systems. The correlation between the structure and stability under pressure for the C-type cubic phase will be discussed followed by the phase transition to B-type or A-type and their coexistence. The results of our DFT computation studies on Dy2O3 which corroborates the experimental results will also be presented.RE2O3 systems exist in three polymorphic forms at ambient conditions, namely: hexagonal A-type, monoclinic B-type, and cubic C-type. They show several structural phase transformations under pressure and many experimental studies have been reported in the recent times. However, the underlying mechanism which determines the stability of the cubic C-type phase for majority of the oxides under pressure and its eventual phase transformation either to the monoclinic or to the hexagonal phase has not been understood. Pressure is expected to promote the stability of A-type over C-type as the molar volume is found to decrease in the phase sequence C-B-A. In this report, the results of high pressure x-ray diffraction studies on a variety of such oxides carried out by our Group will be presented. The systematic structural transition data have been compared and contrasted with the results available for other RE2O3 systems. The correlation between the structure and stability under pressure for the C-type cubic phase w...