COMPOSITE MODULATED STRUCTURES IN ANION EXCESS FLUORITES. Siegbert Schmid, Klaus Fütterer, John G. Thompson and Ray L. Withers, Research School of Chemistry, The Australian National University, Canberra ACT 0200, Australia
Wide range anion-excess, fluorite related solid solutions exist in the yttrium oxide fluoride, zirconium nitride oxide fluoride, zirconium niobium oxide and zirconium tantalum oxide systems, i.e. YO1-[[Delta]]F1+2[[Delta]], 0.12 <= [[Delta]] <= 0.22, (ZrN1-[[Delta]]/2-zF[[Delta]]/2-zO2z)F1+[[elta]], 0.12 <= [[Delta]] <= 0.25, and M2Zrx-2O2x+1 (M = Nb, Ta), 7.1 <= x <= 10.3. All of the above systems can be described as inorganic incommensurate intergrowth compounds or more precisely inorganic misfit layer compounds. Each consists of two chemically different layers that are stacked alternately. In every case one of the layers contains the metals or metals and anions whereas the second layer contains anions only. The widths of these solid solutions are achieved by compressing the anion-only-substructure relative to the other when adding additional anions. It has been shown that such systems are best described as composite modulated structures with a continuously varying primary modulation wave vector of the more strongly scattering metal containing substructure that is linearly dependent on composition (metal to anion ratio). It is this structural flexibility which distinguishes the above systems from other misfit layer compounds that exist as line phases.
It will be shown that the refined atomic modulation functions (AMFs) which are used to describe these systems are remarkably similar despite the variability in composition. Furthermore it will be demonstrated that the attempt to refine structures in these systems as superstuctures often leads to an order of magnitude increase in the number of refined parameters and a rather worse final R-value!
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