High Pressure Phases and Amorphization of a Negative Thermal Expansion Compound TaVO5.

Negative thermal expansion material TaVO5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we have investigated the high pressure phase of TaVO5 using in situ neutron diffraction studies. The first high pressure phase is identified to be monoclinic P21/ c phase, same as the low temperature phase of TaVO5. On heating, amorphous TaVO5 transformed to a new crystalline phase, which showed signatures of higher coordination of vanadium indicating pressure induced amorphization (PIA). PIA observed in TaVO5 might be due to the kinetic hindrance of pressure induced decomposition (PID) into a compound with higher coordination of vanadium. Mechanism of PIA observed in TaVO5 is investigated by carrying out ex situ Raman, XRD, XPS, and XAS measurements. We have also proposed a pressure-temperature phase diagram of TaVO5 qualitatively delineating the phase boundaries between the ambient orthorhombic, monoclinic, and amorphous phases.

Superionic conduction in ß-eucryptite: inelastic neutron scattering and computational studies.

ß-Eucryptite (LiAlSiO4) is known to show super-ionic conductivity above 700 K. We performed inelastic neutron scattering measurements in ß-eucryptite over 300-900 K and calculated the phonon spectrum using classical molecular dynamics (MD) simulations. The MD simulations were used to interpret the inelastic neutron spectra at high temperatures. The calculated diffusion coefficient for Li showed superionic conduction above 1200 K in the perfect crystal. The presence of defects was found to enhance diffusion and lower the temperature for Li diffusion. The calculated trajectory of Li atoms at higher temperatures shows that preferential movement of the Li atom is along the hexagonal c-axis, which is further confirmed by the ab initio calculated activation energy profile for cooperative lithium ion displacements. The inter- and intra-channel correlated motion of Li along the hexagonal c-axis gives the minimum energy pathway for Li ion conduction in LiAlSiO4.

Structural Stability and Anharmonicity of Pr2Ti2O7: Raman Spectroscopic and XRD Studies.

Herein we report results of pressure- and temperature-dependent Raman scattering studies on Pr2Ti2O7. Pressure-dependent studies performed up to 23 GPa suggest a reversible phase transition above 15 GPa with subtle changes. Temperature-dependent investigations performed in the range of 77-1073 K showed anomalous temperature dependence of some of the Raman modes. Temperature-dependent X-ray diffraction data indicated no structural transition but nonlinear expansion of unit-cell parameters with increasing temperature. With increasing temperature, the structure dilates anisotropically, and volume of coordination polyhedra around all the atoms expands. Also with increasing temperature the distortions in coordination polyhedra around all the atoms decrease, and appreciable decrease is observed in Pr(1)O10 and Pr(3)O9 units. The pressure evolution of Raman-mode frequencies was analyzed for both ambient as well as high-pressure phases, and mode Grüneisen parameters for ambient pressure phase were obtained. The temperature evolution of Raman-mode frequencies was analyzed to obtain the explicit and implicit anharmonic components, and it was found that some of the modes attributable to TiO6 octahedra and PrOn polyhedra have dominating explicit anharmonic component. Comparison of the structural data with the temperature dependence of Raman modes suggests that the anomalous behavior in Raman modes is due to phonon-phonon interaction.