Crystalline metastable phase in pressure-cycled epitaxial GaAs

The pressure dependence of the Raman spectrum of epitaxial zinc-blende GaAs has been investigated at room temperature up to 22 GPa. A phase transition to an opaque state is clearly detected at 17.3±0.4 GPa by the disappearance of the zinc-blende phonons. No detectable Raman signal is found in the high-pressure phase. The 1-atm state of GaAs obtained upon reducing the pressure after the phase transition depends on the maximum pressure (Pmax) reached. When Pmax is less than 19 GPa, the zinc-blende phonons reappear, and the observed Raman spectrum indicates an orientationally disordered crystalline phase containing zinc-blende microcrystals 70 in size. When Pmax is greater than 20 GPa, an opaque metastable crystalline phase exhibiting a previously unreported cinnabarlike Raman spectrum is recovered at 1 atm. Possible candidate structures for this metastable phase are discussed in light of the available data.