The effects of pressure on the elastic constants of mercury selenide up to the phase transition

From measurements of ultrasonic wave velocities the elastic constants of mercury selenide, HgSe, have been obtained as a function of hydrostatic pressure up to that (P_t=9.5*10⁸ Pa=9.5 kbar) at which the densification phase transition to the cinnabar structure takes place. At room temperature the elastic constants are at atmospheric pressure C₁₁=62.2, C₄₄=22.7, C'(=¹/₂(C₁₁-C₁₂))=7.9, and at 8.5*10⁸ Pa C₁₁=62.5, C₄₄=22.0, C'=7.37 (units 10⁹ N m^-2). The hydrostatic pressure derivatives are delta C₁₁/ delta P=1.89, delta C₄₄/ delta P=-0.57, delta C'/ delta P=-0.52 and delta B^s/ delta P=2.60. The findings are consistent with a mechanism for the transition which involves macroscopic shear on (110) planes. This suggestion is supported by the negative values obtained for the zone-centre acoustic mode Gruneisen parameters and by the observation that these mode parameters become more negative as pressure is increased. Following a valence force field approach the pressure derivatives of the second-order elastic constants together with an effective ionicity Z(=0.71) have been used to estimate the anharmonic Keating force constants ( gamma =-12.4, delta =-0.82 and epsilon =-1.23 in units of 10¹⁰ N m^-2) and thence a set of third-order elastic constants (C₁₁₁=-21.2, C₁₁₂=-12.7, C₁₂₃=-9.1, C₁₄₄=-0.71, C₁₅₅=-4.3 and C₄₅₆=+0.06 in units of 10¹⁰ N m^-2).