The pseudo-binary mercury chalcogenide alloy HgSe_0.7S_0.3 at high pressure: a mechanism for the zinc blende to cinnabar reconstructive phase transition

The structure of the pseudo-binary mercury chalcogenide alloy HgSe_0.7S_0.3 has been studied by x-ray and neutron powder diffraction at pressures up to 8.5 GPa. A phase transition from the cubic zinc blende structure to the hexagonal cinnabar structure was observed at P ∼ 1 GPa. A phenomenological model of this reconstructive phase transition based on a displacement mechanism is proposed. Analysis of the geometrical relationship between the zinc blende and the cinnabar phases has shown that the possible order parameter for the zinc blende–cinnabar structural transformation is the spontaneous strain e₄. This assignment agrees with the previously observed high pressure behaviour of the elastic constants of some mercury chalcogenides.