Structure determination of complex oxides from X-ray powder patterns by the method of concentration waves (superstructures based on the fluorite lattice)

Abstract

The structures of the complex oxides SC203, Ln2M207, 8c4Ti3012, Sc2VO 5 and Sc9Til0031. 2, which exemplify various types of superstructures based on the fluorite lattice, are discussed. The arrangement of the metal and O atoms and O vacancies over the sites of the basis fluorite matrix is determined from the X-ray powder photographs with a fundamentally new approach to the structure analysis, the concentration-wave method, which provides a means of solving the problem from the observed sets of superstructure reflections, without recourse to their intensities. The results obtained illustrate the potential of the concentration-wave method as applied to interpretation of structures of solids of widely varying chemical composition. Solid solutions having fluorite (CaF2)type structures frequently occur in systems comprising rare-earth metal oxides and oxides of Group IV and V elements. These phases are, as a rule, O deficient and contain O vacancies randomly distributed over the sites of the anionic sublattice. Their composition can be written (M',M)OE_xlq x where M' and M are two sorts of metal ions and [] stands for 0 vacancies. Lowering of temperature results in ordering of the arrangement of metal and 0 atoms and vacancies: a phase transition of the order-disorder type occurs, which manifests itself by the appearance of additional superstructure reflections in the X-ray diffraction pattern. The principal difficulty one encounters in interpreting structures of ordered phases formed from solid solutions arises from the scarcity of available structural information, which is mostly limited to powder photographs. The shortcomings of the powder technique as applied to the solution of superstructures can, however, be overcome to a considerable extent by using the concentration-wave approach. This method makes it feasible to deduce the distribution of atoms from the observed set of superstructure reflections (in certain cases it furnishes a number of possible distribution variants) and obviates the need to analyse the reflection intensities, which are only included at the stage of refinement of the structure parameters.