X-Ray Study of LiCsSO₄ in Connection with Its Ferroelastic Phase Transition

Abstract

The crystal structure of LiCsSO₄ was studied at 25, 0, −20 and −40°C in the normal phase and −71, −78 and −100°C in the ferroelastic phase by using X-ray reflection data collected by an automatic four circle diffractometer. The librational motion of SO₄ is highly anisotropic in the high temperature phase. Temperature variation of the atomic position of S or O atoms was small in the low temperature phase. The mutual proportionality between the temperature dependence of observed structure factors of reflections 201, 203, 205, 311 and 131 and that of the deviation angle Δγ of the monoclinic angle γ from 90° was found, which suggests that the phase transition is characterized by the order-disorder nature of the SO₄ ion coupled with the shear strain x₆.