SYNTHESIS AND CRYSTAL-STRUCTURES OF 2 THALLIUM VANADIUM PHOSPHATES - TLVO2(HPO4) AND TL3V2O3(VO)(PO4)2(HPO4)

Two thallium vanadium phosphate compounds have been crystallized hydrothermally and their structures determined by single-crystal X-ray diffraction. The first, a hydrogen phosphate containing only vanadium(V), crystallizes at 450°C and is orthorhombic, space group Pbca, with a = 9.257(2) Å, b = 17.518(4) Å, and c = 6.810(2) Å; V = 1104.4(5) Å3; Z = 8; Dx = 4.610 g/cm3; R = 0.048; and Rw = 0.048 for 564 independent observed reflections with I ≥ 3σ(I). The structure can be described as chains of corner-sharing distorted square pyramids. Adjacent square pyramids are bridged by the phosphate groups. The chains are linked together by hydrogen bonds to form puckered layers stacked along the b axis. The thallium(I) atoms are approximately 7-coordinated by oxygen atoms in sites between the layers. The compound is isostructural with the MVO2(HPO4) (M = K, Rb, and NH4) compounds whose structures were recently determined by powder X-ray diffraction (1). The second compound, prepared at 200°C, has the composition Tl3V2O3(VO)(PO4)2(HPO4) and contains both V(V) and V(IV). The structure is orthorhombic, space group Pnma, with a = 7.160(2) Å, b = 13.378(2) Å, and c = 14.422(1)Å; V = 1381.4(7) Å3; Z = 4; Dx = 5.378 g/cm3; R = 0.041; and Rw = 0.054 for 1093 independent observed reflections with I ≥ 3σ(I). The structure contains vanadium atoms in two independent sites. The vanadium atom V2 is octahedrally coordinated by six oxygen atoms and forms a OVOVOchain of composition VO(PO4)2 along the crystallographic a direction. The second vanadium atom (V1) is coordinated by five oxygen atoms in a square pyramidal arrangement and is found as a dimeric unit with composition V2O3(HPO4). The two different units are joined to form the complete vanadium phosphate framework structure by sharing the oxygen atoms of one phosphate group. Bond valence calculations indicate that the vanadium atom in the chain is V4+ and that the dimer contains equal amounts of V4+ and V5+. The two thallium atoms occupy distorted 8-coordinate sites in the framework. © 1991.