ON THE STRUCTURAL PHASE-TRANSITIONS IN THE INCOMMENSURATE CS2CDBR4

In the series of incommensurate A2BX4halides with the β-MC2SO4type structure, Cs2CdBr4exhibits an unusual behaviour since the “lock-in” phase transition occurs at the centre of the Brillouin zone. The observed phase sequence is the following: Pnma(Z = 4)⇔INC(k0≈ l/6a*)⇔ P21/n (Z = 4)⇔Pl(Z = 4). These phase transitions have been studied by means of Raman scattering and ultrasonic measurements. It is shown that the Pnma⇔INC⇔P21/n sequence is governed by order-disorder processes due to CdBrJ” tetrahedra reorientations coupled with translations of the Cs+cations, and that the low-temperature P21/n⇔oPl transition is of a displacive nature, governed by a soft optical mode. The “pseudo-proper” ferroelastic character of these transformations is clearly established. A model potential developed in the framework of Landau theory is proposed; this model is able to reproduce the general trends observed in the temperature dependence of the soft-modes and of the elastic constants in the different phases. © 1991, Taylor & Francis Group, LLC. All rights reserved.