A structural study in the system Al₂O₃–WO₃

Abstract

The crystal structure has been determined of a compound initially reported as having the composition 2A1203.5WO3. The final structure analysis proved the substance to be 2AlaO3.6WO3 or A12(WO4)3, and it is isomorphous with the tungstates and molybdates of the smaller trivalent rare-earth elements. A12(WO4)3 crystallizes in the orthorhombic system, space group Pbcn, with lattice constants a= 12.588 + 0.063, b = 9.055 + 0.045, c = 9.127 + 0.046 A. The intensities of three-dimensional equi-inclination Weissenberg data were estimated visually and the structure was solved by Patterson and Fourier techniques. Refinement of atomic parameters was carried out by the method of least-squares. The A12(WO4)3 structure consists of WO4 tetrahedra and A106 octahedra which extend into an infinite three-dimensional network by corner sharing. The average tetrahedral W-O distance is 1.783 (23) A and the average A1-O bond distance is 1-836 (35)A,. The structure contains discontinuous cubic face-centred anion units linked by corner-sharing polyhedra. A high density form of A12(WO4)3 is discussed, based upon a continuous cubic close-packed array of anions.