Ba1−xR x F2 + x Phases (R = Gd-Lu) with distorted fluorite-type structures—products of crystallization of incongruent melts in the BaF2-RF3 Systems (R = Gd-Lu). III. Defect Ba0.75Lu0.25F2.25 structure. A new {Lu8[Ba6F71]} supercluster of defects

Abstract

The structure of Ba0.75Lu0.25F2.25 crystals grown from melt has been studied by X-ray diffraction analysis (4729 measured reflections, 269 independent reflections with I > σ (I), R = 1.1%, R w = 0.7%). The crystals are crystallized in the cubic system, sp. gr. Pm $$\bar 3$$ m, with the lattice parameter a = 5.9870(9) Å. A new complex of defects is singled out—a supercluster of the composition {R 8[Ba6F71]}. This supercluster differs from the well-known rare earth octahedral supercluster of the composition {Ba8[R 6F68-69]} because its nucleus is formed not by RE cations but by an alkali earth cation, Ba. The {R 8[Ba6F71]} supercluster has a configuration close to that of the [B14F64] fragment of the fluorite structure and can replace the latter isomorphously. The model of the Ba0.75Lu0.25F2.25 crystals consisting of coherently intergrown isostructural microphases having different chemical compositions is characterized by the good agreement of the calculated and experimentally determined occupancies of the F1− positions. The comparison of the Ba0.8Yb0.2F2.2 (phase studied earlier) and Ba0.75Lu0.25F2.25 structures demonstrates the evolution of the defect structure along the series of rare earths with the corresponding change of the sp. gr. Fm $$\bar 3$$ m by the sp. gr. Pm $$\bar 3$$ m.