K.K. Pandey

Multiferroic CuCrO2 under high pressure: In situ X-ray diffraction and Raman spectroscopic studies

The compression behavior of delafossite compound CuCrO2 has been investigated by in situ x-ray diffraction (XRD) and Raman spectroscopic measurements up to 23.2 and 34u2009GPa, respectively. X-ray diffraction data show the stability of ambient rhombohedral structure up to ∼23u2009GPa. Material shows large anisotropy in axial compression with c-axis compressibility, κcu2009=u20091.26u2009×u200910−3(1) GPa−1 and a-axis compressibility, κau2009=u20098.90u2009×u200910−3(6) GPa−1. Our XRD data show an irreversible broadening of diffraction peaks. Pressure volume data when fitted to 3rd order Birch-Murnaghan equation of state give the value of bulk modulus, B0u2009=u2009156.7(2.8) GPa with its pressure derivative, B0′ as 5.3(0.5). All the observed vibrational modes in Raman measurements show hardening with pressure. Appearance of a new mode at ∼24u2009GPa indicates the structural phase transition in the compound. Our XRD and Raman results indicate that CuCrO2 may be transforming to an ordered rocksalt type structure under compression.

Pressure induced crystallization in amorphous silicon

We have investigated the high pressure behavior of amorphous silicon (a-Si) using x-ray diffraction and Raman scattering techniques. Our experiments show that a-Si undergoes a polyamorphous transition from the low density amorphous to the high density amorphous phase, followed by pressure induced crystallization to the primitive hexagonal (ph) phase. On the release path, the sequence of observed phase transitions depends on whether the pressure is reduced slowly or rapidly. Using the results of our first principles calculations, pressure induced preferential crystallization to the ph phase is explained in terms of a thermodynamic model based on phenomenological random nucleation and the growth process.

Reinvestigation of high pressure polymorphism in hafnium metal

There has been a recent controversy about the high pressure polymorphism of Hafnium (Hf). Unlike, the earlier known α→ω structural transition at 38u2009±u20098u2009GPa, at ambient temperature, Hrubiak et al. [J. Appl. Phys. 111, 112612 (2012)] did not observe it till 51u2009GPa. They observed this transition only at elevated temperatures. We have reinvestigated the room temperature phase diagram of Hf, employing x-ray diffraction (XRD) and DFT based first principles calculations. Experimental investigations have been carried out on several pure and impure Hf samples and also with different pressure transmitting media. Besides demonstrating the significant role of impurity levels on the high pressure phase diagram of Hf, our studies re-establish room temperature α→ω transition at high pressures, even in quasi-hydrostatic environment. We observed this transition in pure Hf with equilibrium transition pressure Pou2009=u200944.5u2009GPa; however, with large hysteresis. The structural sequence, transition pressures, the lattice parameter...

High pressure phase transition in Nd2O3

High pressure structural and spectroscopic investigations have been carried out on rare-earth sesquioxide, Nd2O3 using energy dispersive x-ray diffraction and Raman scattering techniques. Our measurements show that ambient hexagonal structure of Nd2O3 is stable up to 27 GPa. Beyond this pressure, it undergoes a structural phase transition to a lower symmetry phase. On release of pressure, it reversibly transforms back to the hexagonal phase suggesting the nature of phase transition to be of displacive kind.

Investigating structural aspects to understand the putative/claimed non-toxicity of the Hg-based Ayurvedic drug Rasasindura using XAFS.

XANES- and EXAFS-based analysis of the Ayurvedic Hg-based nano-drug Rasasindura has been performed to seek evidence of its non-toxicity. Rasasindura is determined to be composed of single-phase α-HgS nanoparticles (size ∼24u2005nm), free of Hg(0) or organic molecules; its structure is determined to be robust (<3% defects). The non-existence of Hg(0) implies the absence of Hg-based toxicity and establishes that chemical form, rather than content of heavy metals, is the correct parameter for evaluating the toxicity in these drugs. The stable α-HgS form (strong Hg-S covalent bond and robust particle character) ensures the integrity of the drug during delivery and prevention of its reduction to Hg(0) within the human body. Further, these comparative studies establish that structural parameters (size dispersion, coordination configuration) are better controlled in Rasasindura. This places the Ayurvedic synthesis method on par with contemporary techniques of nanoparticle synthesis.

High pressure iso-structural phase transition in BiMn2O5

The high pressure behavior of multiferroic BiMn2O5 has been investigated using powder x-ray diffraction and Raman scattering techniques as well as density functional theory based first principles calculations. Our investigations show a reversible iso-structural phase transition in BiMn2O5 above 10xa0GPa. The compressibility along the c axis, i.e.xa0along the edge-shared distorted Mn(4+) octahedral chains, has been found to be significantly reduced above this phase transition, suggesting a dominant role of the relatively rigid Mn-O framework in the high pressure phase rather than that of the coordination sphere around the Bi atom. Bader charge analysis of the charge densities obtained from first principles calculations shows partial atomic charge redistribution among Bi(3+) and Mn(3+) atoms across the phase transition which could be the probable cause of this phase transition.

Computer simulations of crystallization kinetics in amorphous silicon under pressure

With the help of computer simulations we have studied the crystallization kinetics of amorphous silicon in solid phase epitaxial (SPE) and random nucleation growth processes. Our simulations employing classical molecular dynamics and first principles methods suggest qualitatively similar behavior in both processes. Pressure is found to reduce the difference in molar volumes and coordination numbers between the amorphous and crystalline phases, which in turn lowers the energy barrier of crystallization. The activation energy for the SPE growth of four coordinated diamond phase is found to reach a minimum (a maximum in growth rates) close to 10 GPa when its density becomes equal to that of the amorphous phase. The crystallization temperatures of successive high pressure phases of silicon are found to decrease, offering a possible explanation for the pressure induced crystallization reported in this material.

Study of pressure induced phase transformation in CTAB capped CdS nanoparticles

Effect of high pressure on as prepared 20mM CTAB capped CdS nanoparticles (size ~4nm) has been analyzed in this paper. Raman scattering has been used to observe the phase transition pressure. X-ray diffraction pattern is used for structural characterization. Raman scattering predicts the phase transition occur from mixed cubical phase to rock salt phase above 6.6 GPa. One of the representative XRD pattern at 9.7 GPa confirms the existence of rock salt phase above 6.6 GPa.

Effect of the surfactant CTAB on the high pressure behavior of CdS nano particles

Angle dispersive x-ray diffraction studies on CTAB capped cadmium sulphide (CdS) nano particles of sizes 70 nm, 44 nm and 10 nm have been carried out up to ~39, ~30 and ~28 GPa respectively. All the three types of nano particles transform to the rocksalt phase at pressures higher than that of bulk CdS. Our analysis of the x-ray diffraction data indicates that the phase transition pressure as well as the bulk modulus of the rocksalt phase of CdS exhibits a non monotonous behaviour as a function of the size of the nano particles. Our high pressure Raman scattering studies on the 10 nm CdS nano particles are in agreement with the x-ray diffraction studies. The presence of the Raman mode in the high pressure rocksalt phase may be attributed to a disorder activated scattering in this phase or to the formation of a core shell structure beyond the transformation pressure.

Phase transition in metal–organic complex trans-PtCl2(PEt3)2 under pressure: insights into the molecular and crystal structure

Structural studies on Pt(II) complexes which have direct correlation with their stereochemistry and microscopic interactions are of immense technological, catalytic and pharmacological importance. Here, we present high-pressure studies on trans-PtCl2(PEt3)2 (Et = C2H5) using infrared (IR) and Raman spectroscopy combined with powder X-ray diffraction (XRD) studies. The ambient structure was solved using single-crystal XRD studies and was further optimized using density functional theory (DFT) simulations. It has been shown that subtle molecular reorientations result in structural phase transition at pressures as low as ∼0.8 GPa. The emergence of Raman active modes in the IR spectra and vice versa indicates the loss of inversion symmetry across the phase transition. The crystal structure of the high-pressure phase has been found to be non-centrosymmetric using XRD studies, which suggest a change in space group from P21/n to P21 across 0.8 GPa. The spectroscopy results also indicate strengthening of inter- and intramolecular C–H⋯Cl hydrogen bonds resulting in a hydrogen bonded supramolecular network. On further compression up to 4.7 GPa, another phase transformation has been detected. The structure was completely retrieved on release of pressure. Thus, the present findings provide sound evidence of intramolecular rearrangements playing a decisive role in tuning bonding and structural characteristics and hence the physicochemical properties of Pt(II) complexes under varying environments.