Jahn-Teller Phase Transitions in RbCuCl3

The phase transition at 66°C in RbCuCl 3 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 P 6 3 / m m c , P 62 c and P 6 3 m c 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 P b c n . 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 M 4 + -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.