ON THE P6(3) MMC TO P6(3)CM PHASE-TRANSITION - APPLICATION TO KNICL3 AND RBFEBR3/

In this paper a model is presented, describing the phase transition of a high-symmetry phase, the hexagonal perovskite (2L) structure, space group P6(3)/mmc, to a low-symmetric phase, P6(3)cm. The model is base d on the hypothesis that the ion size radius ratio of K and Cl (RA+ an d RX-) determines the structural (and dynamical) behaviour of the ABX( 3) hexagonal perovskites. The consequences for a global displacement p attern, resulting from a collapse of the local A cation environment ar e carried through. Among others, the low-symmetry phase P6(3)cm is fou nd. Although the phase transition occurs at K = (2 pi/3a,2 pi/3a, 0), various other points of high symmetry have been subject of considerati on in our analysis. Applications to KNiCl3, RbFeBr3 and TlFeBr3 are pu t forward and discussed.