Research
High Pressure x-Ray Diffraction
High-pressure X-ray diffraction experiments are being done on a variety of intermetallics and inorganic compounds from atmospheric pressure up to ~0.5 Mbar using an indigenously developed megabar diamond anvil cell coupled to a Guinier powder x-ray diffraction system and rotating anode x-ray generator. These experiments provide information on the Pressure-Volume relationship (Equation of State) and pressure induced structural transitions in these materials. Few of these are highlighted here,
Study of f electron based intermetallics (f-IMCs) under pressure
The f-IMCs are interesting because of the unique role the f-electrons of the actinide/lanthanide atoms play in governing their physical and chemical properties. The nature of f-electron states (i.e. whether they are itinerant or localized) depends on the the f-f overlapping and their hybridization with the d and s orbital, which can be tuned by the application of external pressure. Several AB, AB2 and AB3 type f-IMCs have been investigated in our laboratory over the last three decades and several review articles have been published.
Pressure induced structural changes in layered matlockites (BaFX {X=Cl,Br,I})
Matlockites are minerals that crystallize in the PbFCl type layered tetragonal structure (Space group P4/nmm). The scientific interest in these systems stems from both fundamental and technological importance of these. For instance, matlockites like BaFX:Eu (X=Br, Cl, I), find use as x-ray image storage phosphors in imaging plates. From basic physics point of view, it is intriguing that in spite of them being characterized by ionic bonding, they are very soft like van der Waal’s solids, due to the presence of a weakly bonded bilayer of X- layers. In the recent years, we have carried out extensive structural stability studies on the BaFX systems and reported a series of structural transitions in these. These were observed to undergo interesting symmetry lowering transitions from the parent tetragonal structure to a final monoclinic form. Several citations have been made in particular to our paper [Phys.Rev.B, 58, R555 (1998)] detailing the structural behavior of BaFCl.
High Pressure Investigations on Rare-Earth Oxides
The rare-earth (RE) sesquioxides are known to exist in five polymorphic forms, namely, A and H which are hexagonal, B which is monoclinic, and C and X cubic. The range and existence of each phase depends on the ionic radius of the RE and the temperature. High pressure is expected to play an important role in stabilizing different crystal structures. Our recent studies on Gd2O3, Tb2O3 and Ho2O3 reveal interesting structural phase transitions from their cubic structure to hexagonal, monoclinic and monoclinic structures respectively. Further studies on other sesquioxides like Dy2O3 and Er2O3 are being continued.
Exploration of Novel Phases of Materials at Very High P-T conditions
Simultaneous application of high pressure and high temperature can often lead to formation of novel phases of materials that are otherwise not possible at high temperature and atmospheric pressure. Using our recently developed Laser Heated Diamond Anvil Cell (LHDAC) facility, we are currently exploring formation of novel phase of rare-earth carbides and nitrides, which are expected to be super-hard. Also novel III-V, IV-IV and II-VI compounds. Few representative results are,
Synthesis of Novel transition metal carbide
Synthesis of Ruthenium carbide by high pressure - high temperature technique using laser heated diamond anvil cell facility has been carried out. The synthesis is carried out by laser heating a mixture of pure elements, Ru and C at P~7 GPa and T~ 2000K. The temperature quenched sample is characterized by in-situ high pressure x-ray diffraction. Preliminary analysis suggest formation of Ruthenium carbide in hexagonal phase. Also, the high pressure phase is found to be stable in a very narrow pressure range.
Synthesis of Novel IV-IV systems
GexSn1-x has been predicted to be a direct band-gap semiconductor, but attempts to synthesize this in bulk form by conventional synthesis methods have not been successful on account of the poor solubility of Sn in Ge. In this work, laser heated diamond anvil cell (LHDAC) technique has been employed to explore formation of bulk GexSn1-x (x = 0.7) at varying pressures and temperatures. At ~8 GPa, in situ micro-Raman spectroscopy done on several regions of temperature quenched samples laser heated up to ~2000 K reveals vanishing of the intense Ge TO(G) phonon at ~326 cm-1 and appearance of a softer mode, concurrent with appearance of a new high intensity Raman mode at ~660 cm-1. These indicate dilation of the Ge-Ge bond by virtue of significant miscibility of b-Sn at these high P-T conditions and hints at formation of new stiff Ge-Sn bonds.
High Pressure High Temperature Electrical Resistivity
A high pressure–high temperature (hpht) cell in the Bridgman opposed anvil configuration was developed for studying the electrical resistivity behaviour of materials up to 10 GPa and 1300 K. Electrical resistivity and phase transformation behaviour of several systems like metallic glass Fe40Ni40P14B6,, quasi-crystalline Al82Fe18, shape memory alloy Fe-24wt%Mn, TCNE, pure Ni, Fe, Th and U, intermetallics ThAl2, and UAl2 were investigated under hpht conditions. This facility was also used for synthesizing a number of novel phases, such as the polymerized phases of C60 and C70 , high density phase of doped MgB2 and the amorphised phase of the negative thermal expansion coefficient material ZrW2O8.