WTH8ZR18F4O53 - A NEW SUPERSTRUCTURE OF FLUORITE-TYPE, ASSOCIATING ANION-EXCESS AND ANION-DEFICIENT BLOCKS

The crystal structure of a new complex oxyfluoride WTh8Zr18F4O53, a member of a wide series MTh(8)Zr(18)(F, O)(57), was solved on the basis of single-crystal data obtained on a four-circle diffractometer CAD-4 (R = 4.2%). The space group is Fm $($) over bar$$ 3m(Z = 4) and the lattice derives from fluorite-type one by the relationship a = 1578.5 pm = 3 a(F). This structure is characterized by a fee cationic array with full cationic ordering and by an ordered distribution of two kinds of structural units: (i) anion-excess fluorite units containing ''cuboctahedral'' clusters, similar to those observed in anion-excess fluorite phases, e.g., beta-U4O9 and KY3F10 (these units are composed of six ZrO8 square antiprisms, sharing edges with eight ThFO9 polyhedra, about a cuboctahedral anionic cavity occupied by an extra F anion); (ii) Anion-deficient fluorite units, based on the association, about a single WFO5 octahedron, of six ZrO7 and six ZrO8 polyhedra, in a similar way as in ''stabilized'' zirconia and Ln(n)O(2n-2) rare earth oxides (e.g., Pr7O12). Then, WTh(8)ZT(18)F(4)O(53) is the first example of a fluorite superstructure composed of an ordered intergrowth of anion-excess Th8Zr6F3O30 and anion-deficient WZr12FO23 fluorite blocks. WTh8Zr18F4O53 is the anion-excess fluorite superstructure the poorest in anions ever characterized: MX(2.11). The possibility, demonstrated for the first time, to associate in a fluorite structure anion-excess and anion-deficient parts constitutes a conceptual advance in the knowledge of nonstoichiometry. (C) 1995 Academic Press, Inc.