On the structural phase transitions in the incommensurate Cs2CdBr4 |
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| Authors: | V. Rodriguez M. Couzi A. Gomez-Cuevas J. P. Chaminade |
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| Affiliation: | 1. Laboratoire de Spectroscopie Moléculaire et Cristalline (U.R.A. 124-C.N.R.S.) , Université de Bordeaux I , 33405 , Talence Cedex , France;2. I.L.L. , BP 156X, 38042 , Grenoble Cedex , France;3. Laboratoire de Spectroscopie Moléculaire et Cristalline (U.R.A. 124-C.N.R.S.) , Université de Bordeaux I , 33405 , Talence Cedex , France;4. Departamento de Fisica de la Materia Condensada , Facultad de Ciencias, Universidad del Pais Vasco , Apartado 644, Bilbao , Spain;5. Laboratoire de Chimie du Solide du C.N.R.S. , Université de Bordeaux I , 33405 , Talence Cedex , France |
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| Abstract: | ![]()
Abstract In the series of incommensurate A2BX4 halides with the β-K2SO4 type structure, Cs2CdBr4 exhibits 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 ≈ 1/6a*)?P21/n(Z = 4)?P1 (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 CdB2-4 tetrahedra reorientations coupled with translations of the Cs+ cations, and that the low-temperature P21/n?P1 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. |
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| Keywords: | Raman scattering ultrasonic measurements Landau theory phase transitions incommensurate phases Cs2CdBr4 |
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