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

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Abstract
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 (Pt=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 Bs/ 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).

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