Soft TA modes and anharmonicity in cadmium telluride

Abstract

Velocities of ultrasonic modes propagated in the [001] and [110] directions of CdTe, together with their hydrostatic and uniaxial pressure dependences, have been measured at room temperature. The six third-order elastic constants have been evaluated: C ₁₁₁ = −2·13±0·04, C ₁₁₂ = −2·10±0·02, C ₁₂₃ = −0·42±0·04, C ₁₄₄ = 0·14±0·01, C ₁₅₅ = −0·65±0·01, C ₄₅₆ = 0·05±0·01 (in units of 10¹¹Nm⁻²). Both (C ₁₁ - C ₁₂)/2 and C ₄₄ reduce with hydrostatic pressure; all the zone-centre TA modes in CdTe soften in a similar way to those of HgTe and HgSe. This TA mode softening is discussed in relation to the pressure-induced densification phase transition, which takes place at about 35 × 10⁸ Pa. The second- and third-order elastic constants are discussed in terms of a valence-force-field model. Vibrational anharmonicity is further quantified using generalized Grüneisen parameters. Trends in the elastic behaviour of zinc blende structure compounds with respect to Phillips ionicity and the pressure at which the densification phase transition takes place are discussed.