Reinvestigation of the phase transition in ABW-type CsLiSO4: Symmetry analysis and atomic distortions

摘要

ABW-type CsLiSO4 shows a reversible structural phase transition at approximately 202 K, The transformation is translationengleich and the space group symmetries of the two different polymorphic forms are Pnma (phase I) and P2(i)11 (phase II), respectively, The transition was reinvestigated using a group theoretical approach based on the published structural data of the two modifications. A combined rotation of the SO4 and LiO4-tetrahedra was taken as the primary order parameter eta inducing the transformation. Projection operator calculations revealed that the transition can be associated with the irreducible representation Gamma(3)(+) of Pnma. Displacements of the Li, S, and Cs ions as well as the occurrence of certain strain components are symmetry allowed, too, and can couple to eta. The global tilting pattern is compared with the rotational distortions observed in the transitions of ABW-type CsZnPO4. A rigid-unit mode approach is proposed for the explanation of the transitions, (C) 1998 Academic Press. [References: 14]