AMORPHIZATION OF CA-3(VO4)(2) AT HIGH-PRESSURE

The combined X-ray diffraction and Raman spectroscopy data show that C a-3(VO4)(2) irreversibly amorphizes at 10.0 GPa and room temperature. The amorphization, with its onset at about 8.5 GPa, is not preceded by any phase transition in the crystalline phase, which possesses highly anisotropic compressibility (the axial stiffness along the a axis is over 30% larger than the axial stiffness along the c axis). The result s of Raman spectroscopy measurements demonstrate that structural rearr angements in the amorphous Ca-3(VO4)(2) occur between 8.1 and 14.8 GPa . This is associated with a large deformation of the VO43- units or th e increase of the average coordination number for the V5+ cations in t he amorphous material. A large hysteresis of this phase change is obse rved upon decompression, suggesting a first-order character of polymor phic transitions within the amorphous material. (C) 1998 Academic Pres s.