PHASE-TRANSFORMATION OF ALAS TO NIAS STRUCTURE AT HIGH-PRESSURE

The crystal structure of AlAs has been studied in a diamond anvil cell using energy-dispersive x-ray diffraction to 46 GPa. AlAs undergoes a first order phase transformation from zinc blende to NiAs structure. This is the first experimental observation of a III-V compound transfo rming to the NiAs structure. The decrease in volume on transformation is 17 +/- 1 % at 7 GPa. The transformation is reversible but has large hysteresis. The equilibrium transformation pressure is 7 +/- 5 GPa. E quations of state were obtained for both phases. The bulk modulus at z ero pressure, B0, of zinc blende AlAs was measured to be 74 +/- 4 GPa. These results are in good agreement with theoretical calculations.