A COMBINED XAS AND XRD STUDY OF THE HIGH-PRESSURE BEHAVIOR OF GAASO4 BERLINITE

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XR D) experiments have been carried out on alpha-GaAsO4 (berlinite struct ure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two l ocal coordination transformations occur at different pressure rates bu t appear to be coupled. A reversible transition to a high-pressure cry stalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of ar senic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordinatio n occurs at higher pressures and is irreversible.