Stability of rare-earth oxychloride phases:: Bond valence study

The crystal structures of the tetragonal rare earth (RE) oxychlorides, REOCI (RE = La-Nd, Sm-Ho, and Y) were studied by X-ray powder diffraction measurements, Rietveld analyses, and bond valence calculations. The tetragonal structure (space group P4/nmm, No. 129, Z = 2) is stable for all but Er-Lu oxychlorides, which possess a hexagonal structure. The tetragonal structure consists of alternating layers of (REO)(n+)(n) complex cations and Xn- anions, where the rare earth is coordinated to four oxygens and four plus one chlorines in a monocapped tetragonal antiprism arrangement. The Rietveld analyses yielded a coherent series of structural parameters. Preferred orientation and microabsorption effects were found significant. The evolution of interatomic distances and bond angles indicated that the reason for the preferred structure changing from tetragonal to hexagonal is the strain in the chlorine layer. The bond valence parameter B for the RE-O bonds had to be recalculated due to the covalent nature of the (RFO)(n+)(n) unit. The results obtained with the new parameter confirmed the strains in the chlorine layer to be the cause for the phase transition. (C) 2002 Elsevier Science (USA).