F.X. Zhang

Structure change of pyrochlore Sm2Ti2O7 at high pressures

Structural evolution of pyrochlore titanate Sm2Ti2O7 at high pressures was investigated by in situ Raman and x-ray diffraction methods. An intermediate phase was found at high pressures. The structure is a distorted pyrochlore, where anions are disordered after 40GPa while the cations are still somewhat ordered up to 51GPa. When the pyrochlore structure is severely distorted by external pressure, it transforms completely to an amorphous phase quenchable to room conditions.

On the compression behaviour of (Ti0.5,V0.5)2AlC and (Ti0.5,Nb0.5)2AlC to quasi-hydrostatic pressures above 50 GPa

Using a synchrotron x-ray radiation source and a diamond anvil cell we measured the functional dependences of the lattice parameters of (Ti(0.5),V(0.5))(2)AlC and (Ti(0.5),Nb(0.5))(2)AlC to quasi-hydrostatic pressure of the order of 50xa0GPa. Like for other solids in this family of layered ternary carbides and nitrides, the bulk moduli (≈183xa0GPa) are high and no phase transformations were observed. The bulk moduli of the solid solution compositions were lower than those of the end members. The effect was much more dramatic when Ti replaced Nb in Nb(2)AlC than when it replaced V in V(2)AlC. In the former case the modulus drops by 13% from 209xa0GPa, with the softening coming almost exclusively from the c-axis. This somewhat surprising result is ascribed to the corrugation of the Ti-Nb planes in (Ti(0.5),Nb(0.5))(2)AlC. When Ti replaced V in V(2)AlC the drop in modulus was 8%.

Raman studies of Bi2CuO4 at high pressures

The tetragonal compound Bi2CuO4 was investigated at high pressures by using an in situ Raman scattering method. A pressure-induced structural transition started at 20GPa and completed before 37.2GPa was found. Raman measurements revealed that the above phase transition is reversible.

Thermal expansion measurements and the phase transition in the compound GdSi2

The temperature dependence of the structural parameters of orthorhombic silicide GdSi2 was studied by the in situ powder x-ray diffraction method. A structural change from orthorhombic to α-ThSi2-type tetragonal was found at around 818 K. The change in structure with temperature is continuous and reversible without a discontinuous change in volume. The thermal expansion coefficient was calculated from the refined cell parameters. The compound began to react with the residual oxygen in the argon environment and form oxide above 973 K.