Structural studies onA-cation-deficient perovskite-related phases. I. ThNb4O12, thorium/vacancy ordering in slow-cooled samples

Abstract

The A-cation-deficient perovskite, Th0.25NbO3, i.e. ThNb4O12, when slowly cooled from the melt, presents an interesting hierarchy of ordering phenomena. The associated diffraction effects have been studied with electron microscopy/diffraction and X-ray diffraction techniques. Three main types of ordering processes occur, with different degrees of long-range order. A primary ordering of thorium atoms into alternate (001)p planes of A-cation sites (cell ap × bp × 2cp, P4/mmm) exhibits well-established long-range ordering. A secondary ordering of thorium atoms within the (001)p planes (cell 3\sqrt{2\alpha_{p}} × \sqrt{2b_{p}} × 4cp, Immm) is short range in nature and gives rise to superlattice reflections in the form of diffuse rods directed along g(110)p and g(\bar 110)p. The length of the rods corresponds to correlation lengths of only 20-30 A between {110}p planes of thorium atoms, and the thorium/ vacancy ordering is adequately described by a sinusoidal modulation model, with accompanying modulated displacements of niobium and oxygen atoms. Thirdly, a system of octahedral tilts about [110]p or [\bar 110]p axes is described by a cell \sqrt{2\alpha _{p}} × \sqrt{2b_{p}} × 2cp, Pmam. Independent models for thorium/ vacancy ordering and octahedral tilts have been refined with the intensities of the corresponding groups of satellites measured from precession photographs. The complete model for ThNb4O12 can be described in the unit cell 3\sqrt{2\alpha _{p}} × \sqrt{2b_{p}} × 4cp, P2mm. The superlattice reflections arising from the octahedral tilts are split into groups of four satellites in the form of crosses, owing to microdomain formation in ThNb4O12, with domain boundaries parallel to (100)p and (010)p and with average widths of ~25 A.