N. V. Chandra Shekar

Kinetics of pressure induced structural phase transitions—A review

The current status of experimental as well as theoretical advances in the understanding of kinetics of structural phase transitions is reviewed. A brief outline of the classification of phase transitions and classical ideas in the theory of kinetics of phase change is presented first. High pressure experimental techniques developed for studying the kinetics of structural transitions are reviewed and the salient features of each technique is brought out. The experimental technique using the diamond anvil cell (DAC) and image processing gets special mention as it promises to impart a new direction to this field. The usefulness of kinetic parameters in understanding the mechanism of a phase transition is examined. Typical examples from the literature are provided to give a flavour for these kind of studies. In conclusion, several open questions are raised which could pave way for future work in this area.

Performance of a diamond‐anvil high‐pressure x‐ray diffractometer in the Guinier geometry

In this paper, the performance of a novel high‐pressure diamond‐anvil powder x‐ray diffractometer in Guinier geometry is reported. The diffractometer is a combination of a curved quartz‐crystal monochromator, a Mao–Bell‐type diamond‐anvil cell, a flat position sensitive detector, and a Huber goniometer with compound stages to realize this geometry. The incident x‐ray beam is from a rotating anode x‐ray generator. It is shown that the above experimental arrangement results in an enhanced S/N ratio, improvement in resolution, and reduction of the data‐acquisition time. Test results on UAl2 are presented to demonstrate the advantages of the setup in the study of phase transitions at high pressure.

Cubic to hexagonal structural transformation in Gd2O3 at high pressure

High-pressure X-ray diffraction studies on gadolinium sesquioxide (Gd2O3) have been carried out up to a pressure of ∼25 GPa in a diamond-anvil cell at room temperature. Gadolinium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase at around 12 GPa. The high-pressure phase has been identified as a hexagonal La2O3-type structure. The bulk modulus and its pressure derivative of this phase have been calculated.

Crystal structure of UAl2 above 10 GPa at 300 K

Abstract A high pressure X-ray diffraction study on UAl2 has been done up to ≈28 GPa. It undergoes a structural transition at ≈11 GPa and the structure of the high pressure phase has been identified to be of the MgNi2 type with space group P63/mmc. The structure of the ambient pressure phase is of the MgCu2 type with space group Fd3m. From the electron per atom ratio e a , it is expected that it may transform back to the MgCu2 type structure at still higher pressures. On the basis of similar arguments, it is expected that most of the AB2 type Laves phase compounds of the f electron systems with suitable e a ratios may undergo the pressure induced structural transition in the sequence MgCu2 → MgZn2(or MgNi2) → MgCu2 due to increased delocalization of their f electron states.

Pressure-induced metallization of BaMn2As2

The temperature and pressure dependent electrical resistivity rho(T,P) studies have been performed on BaMn2As2 single crystal in the 4.2 to 300 K range upto of 8.2 GPa to investigate the evolution of its ground state properties. The rho(T) shows a negative co-efficient of resistivity under pressure upto 3.2 GPa. The occurrence of an insulator to metal transition (MIT) in an external P ~4.5 GPa is indicated by a change in the temperature co-efficient in the rho(T) data at ~36 K . However complete metallization in entire temperature range is seen at a P~5.8 GPa. High pressure XRD studies carried out at room temperature also shows an anomaly in the pressure versus volume curve around P ~ 5 GPa, without a change in crystal structure, indicative of an electronic transition. Further, a clear precipitous drop in rho(T) at ~17 K is seen for P ~5.8 GPa which suggests the possibility of the system going over to a superconducting ground state.

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.

High Pressure Structural Studies on Rare-Earth Sesquioxides

The Rare-Earth sesquioxides (RE2O3) exhibit interesting physical and chemical properties. Some of these oxides have tremendous technological applications. At ambient conditions, the RE2O3 systems exist in three polymorphic forms, namely: hexagonal A- type, monoclinic B- type, and cubic C- type. The structural stability of three RE2O3 systems: RE = Gd, Ho, Tm and the Tb4O7 system have been studied under high pressure. All the four systems exhibited interesting pressure induced phase transitions. Gd2O3 exhibited C-A transition at ~ 12 GPa, whereas Ho2O3 and Tm2O3 exhibited C-B transition at pressures of 9.5 and 7 GPa respectively. Tb4O7 transformed from its cubic fluorite structure to probably the cotunnite phase at a relatively higher pressure of 27 GPa. The unusual large pressure stability of Tb4O7 was attributed to the presence of Tb4+ ions. The bulk moduli show systematic increase with higher cations, probably due to the enhanced influence of the 4f electron states and increased nature of covalency. However, the mechanisms of the structural transitions C-B-A with increased pressure and increased cation radii are yet to be understood.

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.

Anomalous compression in - an experimental and computational study

High-pressure x-ray diffraction experiments were performed on up to 23 GPa. Anomalous compressibility behaviour of the system was observed in the pressure range 7 - 14 GPa. The experiments are compared with similar observations in and . Band structure calculations have been performed to look for a possible explanation of this behaviour through the concept of electron transfer from the f to the d orbitals.

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.