New Anion-Excess, Fluorite-Related Superstructure Phases in LnF₂-LnF₃, CaF₂-YF₃, and Related Systems

Abstract

Cubic solid solutions in yttrofluorites (Ca,Y,RE)F_2+δ with 0.0 < δ < 0.5 are often described as some of the most representative examples for nonstoichiometry. The classic, so-called Goldschmidt Zintl model with statistical distribution of cations and interstitial anions, however, is nowadays substituted by short-range order models based on neutron powder diffraction analysis. In contrast, long-range order has been revealed by means of single crystal electron diffraction. The observed microdomains are of the same nature as the corresponding superstructure phases. These can be obtained by longtime annealing of the solid solutions, in most cases only at lower temperatures. Earlier, they have been characterized only by means of X-ray powder methods and a systematic procedure for indexing superstructure powder patterns. In the meantime, however, the superstructure geometry of all investigated phases has been confirmed also by single crystal electron diffraction. Presently, six different superstructure types are revealed; all belong to one of the two homologous series, M_mF_2m+5 or M_mF_2m+6, or to both in the case of a phase width. The atomic arrangement of cations and anions is already known for rhβ and rhγ. Only one model exists for the cation distribution in t, rhα, and rhδ. The phenomenon of ordering can be described by two borderline models. In the first and dominant case, the superstructure is determined by a fully ordered cation lattice, and in the second and less important case by anion clusters, which are built up from both interstitial and shifted parent structure anions. These anion clusters, however, are significantly more extended than the so-called Willis cluster.