Ab initio structure determination of lanthanum cyclo-tetratungstate α-La2W2O9 from X-ray and neutron powder diffraction

Lanthanum tungstate alpha -La2W2O9 has been successfully synthesized by the ceramic route. Its crystal structure was determined ab initio from neutron diffraction data (space group P1 No. 2; a = 7.2489(1) Angstrom, b = 7.2878(1) Angstrom, c = 7.0435(1) Angstrom, alpha = 96,367(1)degrees, beta = 94,715(1)degrees, gamma = 70.286(1)degrees; Z= 2; reliability factors: R-p = 6.0%, R-wp = 6.9%, R-B = 3.0%). The structure exhibits isolated [W4O18](12-) groups in the form of rings involving [WO6] octahedra alternating with [WO5] trigonal bipyramids sharing corners. These rings are inserted in the 3D framework built by the 9- and 10-fold coordination polyhedra of the lanthanum atoms. (C) 2001 Academic Press.