LU3O2F5 - A NEW HIGHLY DENSIFIED MEMBER (N=3) OF THE M(N)X(2N+1) SERIES OF FLUORITE-RELATED VERNIER PHASES

In addition to the nonstoichiometric LuO1-xF1+2x (0.11 less than or equal to x less than or equal to 0.22) domain of incommensurate vernier phases, a new stoichiometric phase Lu3O2F5 (X = 0.33) has been isolated. Its crystal structure has been solved by a Rietveld analysis of its powder X-ray pattern (R(B) = 0.047). Lu3O2F5 crystallizes with orthorhombic symmetry (space group: Pnma) and the unit cell parameters a = 5.544(1) Angstrom, b = 16.875(3) Angstrom, c = 5.292(1) Angstrom and Z = 4. Its crystal structure can be described as the commensurate member n = 3 of the M(n)X(2n=1) series of family I vernier phases, and exhibits the highest anionic rate ever observed in such phases (MX(2.33)) The crystallochemical factors justifying such a surprising anionic densification are analyzed and emphasized. A comparison with Bi3NF6, whose structure corresponds to the member n = 3 of the series of family II vernier phases, is developed. (C) 1995 Academic Press, Inc.