We report dramatically different behaviors between isostructural Gd2Ti2O7 and Gd2Zr2O7 pyrochlore at pressures up to 44 GPa, in which the substitution of Ti for Zr significantly increases structural stability. Upon release of pressure, the Gd2Ti2O7 becomes amorphous. In contrast, the high-pressure phase of Gd2Zr2O7 transforms to a disordered defect-fluorite structure. First-principle calculations for both compositions revealed that the response of pyrochlore to high pressure is controlled by the intrinsic energetics of defect formation.

An irreversible structural transformation from the cubic phase to a hexagonal high-pressure phase was verified in Gd 2 O 3 between 7.0 and 15 GPa. The compressibility and bond distances of both phases were determined by the refinement of the x-ray diffraction data. The high-pressure phase of Gd 2 O 3 is 9.2% denser than the cubic phase at 7 GPa. After release of pressure, the high-pressure phase transformed to a monoclinic structure. The pressure-induced phase transition from the monoclinic to the hexagonal phase is reversible. Unlike the case at atmospheric pressure, the hexagonal phase was found to transform to the monoclinic phase by increase of temperature at high pressures. The lattice potential energies and electronic density of states of the cubic, monoclinic, and hexagonal high-pressure phases of Gd 2 O 3 were calculated from the known structural models with density-functional method. The observed phase stability, transition pressure, and volume change are well explained by theoretical calculations.

High-Pressure U 3O8 with the Fluorite-Type Structure. -A dense fluorite-like form of U 3O8 is prepared from -U3O8 at pressures greater than 8.1 GPa. The high-pressure phase crystallizes in the space group Fm3m (in situ XRD) and is stable at pressures at least up to 40 GPa and temperatures up to 1700 K. It is quenchable to ambient conditions. The quenched high-pressure phase is 28% denser than the initial orthorhombic phase at ambient conditions. -(ZHANG*, F. X.; LANG, M.; WANG, J. W.; LI, W. X.; SUN, K.; PRAKAPENKA, V.; EWING, R. C.; J. Solid State Chem. 213 (2014) 110-115, http://dx.