The structure of germanium niobium oxide, an inherently non-stoichiometric ‘block’ structure

Abstract

Germanium niobium oxide, reported as Ge02. 9Nb20 5, is inherently nonstoichiometric since it appears to be isostructural with P 20 5. 9Nb20 5. To fit this structure, there must be vacant oxygen sites or some sites accommodating ‘interstitial’ metal atoms, in relatively high concentration, and the mode of incorporating a stoichiometric excess of cations should cast some light on other niobium oxide type structures which have a reported range of composition. The structure of germanium niobium oxide has been determined by a combination of three methods: lattice imaging electron microscopy, to establish that the non-stoichiometry was not attributable to extended defects; neutron diffraction, using the powder method and profile analysis, for particular evidence about the anion sublattice and distribution of cations; and X-ray diffraction, for an ab initio refined structure. It has been proved that the anion lattice is essentially complete, and that the cation excess is accommodated by inserting cations into a set of sites, with distorted octahedral coordination, in the square tunnels formed by the junctions between columnar elements of structure. Occupation of these octahedral sites precludes the occupation of adjacent tetrahedral cation sites, proper to the type structure. In consequence, there are constraints on the way that the two kinds of tunnel site can be occupied to produce the observed stoichiometric excess of cations. The resulting model can be generalized to interpret the metal-excess composition ranges found for other niobium oxide structures.