The hydrostatic pressure derivatives of the elastic constants of indium and an indium cadmium alloy

Abstract

The hydrostatic pressure derivatives of the elastic stiffness constants of indium and indium-3.4 at.% cadmium alloy single crystals have been obtained from pulse echo overlap measurements of the dependence of ultrasonic wave velocities upon pressure. The softest zone centre acoustic phonon mode in indium is a shear mode propagating k along the [101] direction rather than that ( k[110], e[11̄0]) which drives the ferroelastic phase transition in the indium-cadmium alloys. The derivative δ(( C 11 – C 12)/2)/δ P is positive, accounting for the stability of the fct structure of indium under high pressure. Using the quasiharmonic, anisotropic continuum model the acoustic mode Grüneisen parameters have been calculated and are discussed in terms of mode softening. The high temperature limiting value $ ̄ γ H (= 2.56) of the mean acoustic mode Grüneisen parameter is found to be close to the thermodynamic Grüneisen parameter γ th (=2.5).