The crystal structure and spectral properties of bulk MoS 2 were investigated at high pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high temperature X-ray diffraction and high pressure Raman spectroscopic analysis. While the crystal structure of MoS 2 is stable on increasing temperature, results of high pressure experiments show a pressure-induced isostructural hexagonal distortion to a 2H a -hexagonal P6 3 /mmc phase around 26 GPa as predicted by theoretical calculations reported earlier. The 2H a -hexagonal phase coexists with the ambient 2H c phase up to 51 GPa, the highest pressure achieved in our experiments. The Raman data obtained in our high pressure experiments show consistent changes in the vibrational modes. Furthermore, the diffraction data obtained for the shocked MoS 2 to pressures 8 GPa is found to be structurally resilient.

Orthosilicates adopt the zircon structure-types (I41/amd), consisting of isolated SiO4 tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development and material science. Stetindite (CeSiO4) and coffinite (USiO4) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 °C to ~850 °C. Stetindite maintains its I41/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high temperature Raman and FTIR spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C, and then thermally decompose into a mixture of UO2 and SiO2. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high temperature structures of two coffinite-thorite solid solutions, uranothorite (UxTh1-xSiO4), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO4, USiO4, U0.46Th0.54SiO4, and U0.9Th0.1SiO4 were determined to be αV = 4.21 × 10 -6

Structural behavior of bulk WS 2 under high pressure was investigated using synchrotron X-ray diffraction and diamond anvil cell up to 52 GPa along with high temperature X-ray diffraction and high pressure Raman spectroscopy analysis. The high pressure results obtained from X-ray diffraction and Raman analysis did not show any pressure induced structural phase transformations up to 52 GPa. The high temperature results show that the WS 2 crystal structure is stable upon heating up to 600 • C. Furthermore, the powder X-ray diffraction obtained on shock subjected WS 2 to high pressures up to 10 GPa also did not reveal any structural changes. Our results suggest that even though WS 2 is less compressible than the isostructural MoS 2 , its crystal structure is stable under static and dynamic compressions up to the experimental limit.

We have investigated the structural behavior of CuSbS 2 and CuSbSe 2 thermoelectric materials under high pressure conditions up to 80 GPa using angle dispersive x-ray diffraction in a diamond anvil cell (DAC). We have also performed high-pressure Raman spectroscopy measurements up to 15 GPa. We observed a pressure-induced structural transformation from the ambient orthorhombic structure with space group Pnma to a triclinic type structure with space group P1 beginning around 8 GPa in both samples and completing at 13 GPa and 10 GPa in CuSbS 2 and CuSbSe 2 , respectively. High pressure Raman experiments complement the transitions observed by high-pressure x-ray diffraction (HPXRD). The transitions were found to be reversible on releasing the pressure to ambient in the DAC. The bulk modulus and compressibility of these materials are further discussed.

We have investigated the crystal structure and magnetic ordering of LaFeAsO at low temperature (∼10 K) and high pressures. The long range antiferromagnetic ordering is suppressed under pressure, and the structural parameters obtained show a close correlation between the anionic height (ha) and the transition temperature (Tc). An orthorhombic to tetragonal transition is observed above 30 GPa. Density functional theory calculations show that the shape of the hole surface becomes two dimensional under pressure. Our results demonstrate that the variation of ha and dimensionality play important roles in the evolution of pressure induced superconductivity in addition to magnetic ordering.