ER2BANIO5 - STRUCTURE REFINEMENT USING NEUTRON POWDER DIFFRACTION DATA

The crystal structure of dierbium barium nickelate, Er2BaNiO5, has been refined from neutron powder diffraction data, collected at room temperature. Results agree with those obtained from single-crystal X-ray diffraction. Sixfold oxygen coordination around the Ni atoms is confirmed and more precise oxygen positions are given. The NiO6 octahedra show an unusual twofold distortion: the Ni-O distances to the two axial O atoms are considerably shorter, by 0.3 angstrom, than those to the four equatorial oxygens; these oxygens are distorted from the right angles of a regular octahedron to 79.1 (1)-degrees. Interesting physical properties are suggested by the existence of infinite isolated chains of NiO6 flattened octahedra sharing vertices in the direction of the a axis, with extremely short, 3.75 angstrom, Ni-O(axial)-Ni distances. Orthorhombic, Immm, a = 3.7541 (1), b = 5.7442 (1), c = 11.3019 (2) angstrom, V = 243.71 (2) angstrom 3, Z = 2, lambda = 1.594 angstrom, R(wp) = 7.52%.