Theoretical study of pressure-driven phase transitions in HgSe and HgTe

We present the results of a density-functional-theory study of the stability of high-pressure phases of HgSe and HgTe up to the so-called Cmcm phase, which is the highest-pressure phase that has been fully characterized in experiments in these compounds. We compare the results obtained using different exchange-correlation functionals for the energetics, stability, and structure of the high-pressure phases, finding that the qualitative picture is very similar and essentially in agreement with the experimental observations. We study the cinnabar -> NaCl and the NaCl -> Cmcm transitions in detail and provide the structural evolution under pressure of the cinnabar and Cmcm phases. We find that the mechanism for the NaCl -> Cmcm transition is somewhat different from other related compounds exhibiting this transition, with cell deformation playing an important role in the stabilization of the Cmcm state in HgSe and HgTe. Our results support the view of a NaCl -> Cmcm transition that is distinctly first order, in good agreement with the experimental observations and contrary to other materials where this transition is of second order.