P. Ch. Sahu

The pressure-amorphized state in zirconium tungstate: a precursor to decomposition

In contrast to widely accepted view that pressure-induced amorphization arises due to kinetic hindrance of equilibrium phase transitions, here we provide evidence that the metastable pressure-amorphized state in zirconium tungstate is a precursor to decomposition of the compound into a mixture of simple oxides. This is from the volume collapse ΔV across amorphization, which is obtained for the first time by measuring linear dimensions of irreversibly amorphized samples during their recovery to the original cubic phase upon isochronal annealing up to 1000 K. The anomalously large ΔV of 25.7 ± 1.2% being the same as that expected for the decomposition indicates that this amorphous state is probably a precursor to kinetically hindered decomposition. A P–T diagram of the compound is also proposed.

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.

Pressure induced dimerisation of C70

Solid C70 has been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 750°C. X-ray diffraction measurements on the recovered samples indicate that the initial h.c.p. solid C70 transforms to a rhombohedral structure which recovers to an f.c.c. structure on annealing. The associated changes in the intra molecular vibrational modes have been probed through infrared (IR) and Raman measurements. The IR measurements on these HPHT samples show splitting of some of the pristine modes and occurrence of several new modes. These sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been attributed to the formation of C70 dimers.

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 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.

Phase transformation and electrical resistivity of tetracyanoethylene under pressure

This paper reports the phase transformation behaviour of tetracyanoethylene (TCNE) under pressure as revealed by AC electrical resistivity, its time evolution and X-ray diffraction studies. An irreversible transformation from monoclinic to cubic phase occurs at 2.1±0.1 GPa and is indicated by a sharp resistivity drop at this pressure. The time evolution of resistivity studies indicate that this transformation occurs via an intermediate phase having resistivity higher than either of the two crystalline phases. Finally, the kinetics of phase transformations obtained by time evolution of resistivity is compared with the X-ray studies on the pressure quenched TCNE.