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

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.