STRUCTURAL CHARACTERIZATION AND POLYMORPHISM OF R2BANIO5 (R = ND, GD, DY, Y, HO, ER, TM, YB) STUDIED BY NEUTRON-DIFFRACTION

The family of compounds R2BaNiO5 (R = Nd, Dy, Y, Ho, Er, Tm, Yb) has been studied by neutron powder diffraction at room temperature. All of them present, orthorhombic symmetry(space group Immm, Z = 2). The main feature of these materials is the existence of one-dimesional chains of vertex-sharing NiO6 octahedra along the a-axis. The present research has revealed the effect of the size of the rare earth ions on the structural parameters, which plays a role nearly equivalent to a hydrostatic “chemical pressure” in the geometric distortion of the structure. The volume or the Ni and Ba polyhedra varies linearly vs the ionic radius of the rare earth. The overall agreement between the valence bond sums and formal charges is very poor. This fact indicates that the present structure is under very high internal stress and may be metastable. We speculate that there may exist a dimorphism in the other members of this family of oxides, similar to that already found for the Tm, Yb, and Lu compounds. © 1993 Academic Press, Inc.