  The structure of the pseudo-binary mercury chalcogenide alloy HgSe0.7S0.3 has
been studied by means of 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 the displacement
mechanism is proposed. The 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.
