Elastic anomalies in the high-temperature cubic phase of some A₂BX₆ compounds

Abstract

A comparative study using Brillouin scattering and ultrasonic experiments has been made of the temperature dependence of the elastic constants of a number of A₂BX₆ compounds. Special emphasis has been placed on effects arising from lattice instabilities near incipient structural phase transitions which occur in a number of these antifluorite structure compounds. Measurements have been made in the temperature range 4·2-445 K on single crystals of K₂SnCl₆, (NH₄)₂SnCl₆, K₂ReCl₆, (NH₄),SnBr₆, Rb₂PtBr₆, K₂TeBr₆, (NH₄)₂TeBr₆ and K₂SeBr₆. In the cubic phase of those crystals which do undergo phase transitions, a marked softening of either one or both shear elastic constants C ₄₄ and (C ₁₁− C ₁₂)/2 has been observed which has been analysed quantitatively in the framework of an acoustic mode-optical mode coupling model. The effects of hydrostatic and uniaxial pressure on the elastic constants can also be predicted on the basis of this model. To test the model predictions, the behaviour with temperature of the effects of pressure on the renormalization contribution to the elastic constants have been examined in K₂SnCl₆.