Soft-phonon instability in zincblende HgSe and HgTe under moderate pressure: Ab initio pseudopotential calculations

The mercury chalcogenides HgSe and HgTe show peculiar behavior under moderate compression such as the unusual observation of a “hidden” transition from their ambient-pressure zincblende phase to an orthorhombic metastable phase with space group C222₁. We present here the results of an ab initio pseudopotential study within the framework of the density-functional theory of the stability of the low-pressure zincblende phases of HgSe and HgTe and the transition to the C222₁ structure. We relate the observation of the C222₁ phase to an instability in the transverse-acoustic phonon branches of the zincblende phases of these materials. We have studied in detail the mechanism of the zincblende→C222₁ transition as well as the structural evolution of the C222₁ phase under pressure.