V. Kathirvel

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.

Si doped T6 carbon structure as an anode material for Li-ion batteries: An ab initio study

First-principles calculations are performed to identify the pristine and Si doped 3D metallic T6 carbon structure (having both sp2 and sp3 type hybridization) as a new carbon based anode material. The π electron of C2 atoms (sp2 bonded) forms an out of plane network that helps to capture the Li atom. The highest Li storage capacity of Si doped T6 structure with conformation Li1.7Si1C5 produces theoretical specific capacity of 632 mAh/g which substantially exceeding than graphite. Also, open-circuit voltage (OCV) with respect to Li metal shows large negative when compared to the pristine T6 structure. This indicates modifications in terms of chemical properties are required in anode materials for practical application. Among various doped (Si, Ge, Sn, B, N) configuration, Si doped T6 structure provides a stable positive OCV for high Li concentrations. Likewise, volume expansion study also shows Si doped T6 structure is more stable with less pulverization and substantial capacity losses in comparison with graphite and silicon as an anode materials. Overall, mixed hybridized (sp2 + sp3) Si doped T6 structure can become a superior anode material than present sp2 hybridized graphite and sp3 hybridized Si structure for modern Lithium ion batteries.

CORRELATION BETWEEN STRUCTURAL STABILITY AND ELECTRONIC STRUCTURE OF UGa3 UP TO 30 GPa

High-pressure angle-dispersive X-ray diffraction experiments were performed on UGa3 up to 30 GPa within a diamond-anvil cell. UGa3 remains in its cubic AuCu3 type structure up to the maximum pressure studied and does not show any structural phase transition. To understand the structural stability of UGa3, band structure calculations were performed as a function of reduced volume using the full-potential linear augmented plane wave (FP-LAPW) method. The results show that the Fermi level coincides with a deep valley in the density of states (DOS) curve in the antiferromagnetic state, whereas it lies near a valley (towards the bonding side) in the nonmagnetic state. At high pressures, the DOS near EF does not show much variation in both the cases. The experimental and theoretical equation of state, bulk modulus, and its pressure derivative values are also reported. The pressure dependence of magnetic moment shows a linear decrease at the rate of dμ/dP = -0.027 μB/GPa.

Electronic structure of URh3 up to 40 GPa

Full potential linear augmented plane wave (FP-LAPW) method is used to calculate the electronic structure of URh3 in its AuCu3 type structure as a function of reduced volume. The Fermi level lies near a maxima in the DOS and it remains pinned at the same position even at 40 GPa. The charge density plots show that the charge distribution becomes more uniform, the directional bonding decreases at very high pressure and the system becomes more metallic. The present analysis suggests that the AuCu3 structure might remain as the ground state for URh3 up to quite high pressures. A 2-D structural stability map constructed for the AB3 type system indicates a wide structural stability regime for URh3 under high pressure.