Polymorphism in the oxides R2BaNiO5 (R=Tm, Yb, Lu)
Abstract The family of compounds R 2 BaNiO 5 for R=Nd, Dy, Y, Ho and Er are orthorhombic, Immm, whereas for R=Tm, Yb, Lu two different orthorhombic phases can be stabilized, Immm and Pnma, depending on the synthesis conditions. The main feature of the Immm-phase is the existence of one-dimensional chains of vertex-sharing NiO 6 octahedra along the a -axis. The Pnma-phase is isomorphous to the so called “green phase” Y 2 BaCuO 5 , where the transition metal occupies isolated pyramidal-coordinated sites in the structure. The overall agreement between the valence bond sums and formal charges is very poor for the case of the Immm-phase for all the members of the family, indicating that this structure is under very high internal stress. The dimorphism found for the Tm, Yb and Lu compounds is discussed in the light of the preliminary high temperature X-ray diffraction experiments.