The structure of the A-cation-deficient perovskite UNb4O12

The compound UNb4O12, when prepared by slow cooling in vacuum from 1573 K, displays a pseudotetragonal perovskite superstructure. Electron-microscopy studies show it has a microdomain structure in the ab plane and the apparent tetragonal symmetry is due to interchange of a and b of an orthorhombic structure in domains of size proportional200 A. The structure was determined by a method of successive approximations, using different groupings of the superlattice reflections that could be indexed by subcells of the 4x4x4 superlattice and could be associated with particular structural features. A close approximation to the average structure in a single domain was obtained in a 4x2x4 supercell, space group Immm. It comprises a face-centred array of [010] columns of U atoms in alternate (001) planes. The column ordering along [100] is not fully developed and can be described by a sinusoidal modulation. The framework of NbO6 octahedra distorts to reduce the coordination number and U-O bond lengths of the U atoms from the cuboctahedral coordination of ideal perovskite. The anion displacements may be resolved into an octahedral tilt component about [101] axes plus a displacement component due to the U/vacancy distribution. The octahedral tilt system may be described as twinned a-b0c- of magnitude proportional60. Regular twinning of the tilted octahedra on (100) and (001) planes gives rise to a characteristic splitting of the tilt reflections. (orig./TWO)