BERLINITES UNDER PRESSURE

Berlinites, compounds with the generic formula ABO(4), form a class of materials isostructural to quartz, with a doubling of the unit cell a long the c-axis. The A atoms are either Al or Ga and B are P or As. Th ese materials were studied as a function of pressure by x-ray diffract ion and x-ray absorption, and some of them by Brillouin and Raman scat tering. The ambient condition quartz structure is destabilized by the pressure, and transition(s) to denser phases, either amorphous or crys talline, are observed. The structure of the high pressure phase has be en solved for alpha-GaPO4. It is orthorhombic and belongs io the Cmcm or a closely related space group, like the high pressure phase of cris tobalite form of this compound. The room temperature phase diagram of these compounds is discussed.