REFINEMENT OF THE STRUCTURE AND COMPOSITION OF THE LAYERED PYROCHLORE-LIKE OXIDE CS6+XW11O36

The symmetry, composition, and crystal structure of Cs6.3W11O36 (a = 7 .2742, c = 110.79 Angstrom, Z = 6, R(3) over barc$, R = 0.057), previo usly considered monoclinic and stoichiometric, are refined The tungste n-oxygen octahedral units of the structure, which have an AB(2)O(6) de fective pyrochlore motif, lie in layers perpendicular to the c axis, w ith Cs+ ions located in the voids of layer units and between them. An additional interlayer position of cesium, occupied by 31(8)%, has been found. The structures of Cs8.5W11O48 and defective pyrochlore, relate d to Cs6+xW11O36, form a homologous series with the general formula A( 2n)A'(1-y)M(4n-1)X(12n) (for the above members, n = 3, 4 and infinity, respectively); the members in the series differ in the number of stor eys in pyrochlore-like layers.