Observation of a Cinnabar Phase in GaAs at High Pressure

A previously unidentified high-pressure phase of GaAs has been found on pressure decrease from GaAs-II. The new phase is hexagonal, with a fourfold coordinated cinnabar structure very similar to that of ZnTe-II. The phase first appears on download from GaAs-II at 11.9(1) GPa, coexists with GaAs-I below 11.2(3) GPa, and starts to transform to GaAs-I at 8.1(2) GPa. Although no evidence of the phase is seen on pressure increase, detailed comparisons with ZnTe suggest cinnabar GaAs is an equilibrium phase.