Z.W. Hendrikse, W.J.A. Maaskant On the P6_3/mmc to P6_3cm phase transition: Application to KNiCl_3 and RbFeBr_3 Physica B 233 1997 139-152 Zeger W. Hendrikse Structural and dynamic modelling on some ABX_3 hexagonal perovskites Rijksuniversiteit Leiden 1997 April