Defect structure of anion-excess fluorite-related Ca1−xYxF2+x solid solutions

On the basis of the transformation of a cube within a fluorite-type matrix into an archimedean antiprism of the kind found in numerous ordered anion-excess fluorite-related superstructures, a new polyhedral cluster, labeled 4:4:3, according to B. T. M. Willis, Proc. Br. Ceram. Soc.1, 9 (1964) and A. K. Cheetham, B. E. F. Fender, and M. J. Cooper, J. Phys. C4, 3107 (1971)], is proposed to explain the defect structure and short-range order in Ca_1?xY_xF_2+x solid solution. In agreement with the spectroscopic, dielectric, and electric experiments, this new structural model fits perfectly well the measured occupation numbers for normal F and interstitial F′ and F″ fluorine atoms for the whole range of compositions without requiring the too short F′F′ distances generated by the previously proposed 2:2:2 or 3:4:2 clusters. Such 4:4:3 clusters and nearly identical 4:4:4 or 4:4:5 ones, are probably present in the other highly and moderately doped Ca_1?xLn_xF_2+x solid solutions; they could be precursors for the largest clusters, i.e., ordered microdomains, observed for samples annealed for a long time.