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 transforming to the NiAs structure. The decrease in volume on transformation is 17[plus minus]1% at 7 GPa. The transformation is reversible but has large hysteresis. The equilibrium transformation pressure is 7[plus minus]5 GPa. Equations of state were obtained for both phases. The bulk modulus at zero pressure, [ital B][sub 0], of zinc blende AlAs was measured to be 74[plus minus]4 GPa. These results are in good agreement with theoretical calculations.