NEW MIXED-VALENCE COMPOUNDS IN THE PB-V-O SYSTEM - SYNTHESIS AND CRYSTAL-STRUCTURE OF HOLLANDITE-RELATED PB1.32V8.35O16.7 AND R-TYPE HEXAGONAL FERRITE PBV6O11

Two new mixed valence vanadium oxides Pb1.32V8.35O16.7 and PbV6O11 hav e been discovered in the Pb-V-O system. Their crystal structures were determined at room temperature by single-crystal X-ray analysis. Pb1.3 2V8.35O16.7: monoclinic, I2/m, 10.108(3), b = 9.887(3), c = 2.903(1) A ngstrom, gamma = 90.84(2)degrees, and Z = 1, final R = 0.049 for 452 i ndependent refections. PbV6O11: hexagonal, P6(3)mc, a = 5.754(1), c = 13.267(3) Angstrom and Z = 2, final R = 0.035 for 353 independent refl ections. For Pb1.32V8.35O16.7, V(1) and V(2) octahedra form a hollandi te-type framework. Pb(1) and Pb(2) alternate randomly with O(5) atoms in the hollandite-type tunnels which are fully occupied. The O(5) atom s create additional oxygen tetrahedra occupied by intratunnel V(3) ato ms. The structure of PbV6O11 is closely related to BaTi2Fe4O11 and AV( 6)O(11) compounds (A = Na, Sr) which adopt an R-type hexagonal ferrite structure: O and Pb atoms form a hexagonal close-packed structure, an d V(1), V(2), and V(3) atoms are surrounded octahedrally by six oxygen atoms. V(2)O-6 and V(3)O-6 octahedra are face-shared and occupied by V3+ and V4+ ions, respectively, whereas they are equivalent in other A V(6)O(11) compounds. The lack of symmetry due to the lone pair effect of the Pb2+ ion is responsible for the split of these vanadium atoms. The Pb2+ ion is displaced from the center of the cuboctahedron to a ba sal O-3 triangle and forms an umbrella-like PbO3 entity. As a conseque nce, the V(4) atom is also off-centered in the trigonal-bipyramid and its environment is rather better described as a tetrahedron, like Fe3 in numerous magnetoplumbite-type compounds. (C) 1996 Academic Press, Inc.