After a pressure decrease to ambient, the high-pressure SC16 phase of GaAs is found to transform to the hexagonal wurtzite structure. This has been suggested for GaAs in calculations but never previously observed experimentally. Wurtzite-GaAs is found to be stable at ambient pressures at temperatures up to 473 K, with a structure that is only slightly distorted from ideal. On recompression, the $c/a$ ratio is constant with pressure and wurtzite-GaAs transforms to the orthorhombic $Cmcm$ phase at 18.7(9) GPa.

• Received 10 June 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.215505