Jahn-Teller Phase Transitions in RbCuCl₃

Abstract

The phase transition at 66°C in RbCuCl₃ has been investigated by X-ray diffraction methods. Structure analyses have been carried out using the photographic data. For high-temperature structure, three equally plausible space groups P6₃⁄mmc, P\bar62c and P6₃mc have been examined by a block diagonal least squares refinement. The room-temperature structure has the superstructure with the lattice parameter doubled along the b-axis and the space group is Pbcn. The mode analysis of the Jahn-Teller active mode was carried out and the intensity distribution of the superlattice reflections was consistent with the atomic shifts corresponding to the zone boundary M₄⁺-mode. There exists the second-order piezodistortive coupling between the antiferrodistortive static displacements and the homogeneous strains. From this coupling, the precise temperature dependence of the superlattice reflection is derived.