Elastic properties of Rb₄LiH₃(SO₄)₄and K₄LiH₃(SO₄)₄

Abstract

High-resolution Brillouin spectroscopy was used to study the elastic properties of Rb4LiH3(SO4)4 and K4LiH3(SO4)4 crystals. In contrast to earlier studies of the former compound, the present results indicate that the ferroelastic phase transition, which occurs at approximately 115 K, is of the 4 to 2 type rather than 4 mm to mm2 as previously believed. An appropriate theoretical model using a two-component order parameter free energy expansion has been presented based on the Landau theory. K4LiH3(SO4)4 does not appear to be ferroelastic in the temperature range examined and only exhibits a slight change in slope of the temperature plots for elastic constants associated with longitudinal phonons.