CHAINS OF EDGE-SHARING OPb4 TETRAHEDRA IN THE STRUCTURE OF Pb4O(VO4)2 AND IN RELATED MINERALS AND INORGANIC COMPOUNDS

The crystal structure of Pb4O(VO4)2 has been solved using crystals prepared by high-temperature solid-state reactions. The compound is orthorhombic, Pnma, a 22.7276(9), b 11.5346(4), c 7.4349(3) Å, V 1949.09(13) Å3, Z = 8. The structure was solved by direct methods and refined on the basis of F2 of all unique reflections to R1 = 0.029 (wR2 = 0.052), calculated for 3052 unique observed reflections. There are six symmetrically independent Pb2+ cations in the structure of Pb4O(VO4)2. The Pb–O bonds in the PbOn polyhedra range from 2.23 to 3.42 Å, and can be subdivided into three groups: short bonds (2.15–2.50 Å), intermediate bonds (2.50–2.70 Å) and long bonds (>2.70 Å). The Pb2+ cations each form from three to five short and intermediate Pb–O bonds located on one side of the cation, and from three to seven long Pb–O bonds on the other side of the cation. The asymmetry of the distribution of bonds around the Pb2+ cations is due to the stereochemical activity of 6s2 lone-electron pairs. The structure contains chains of composition [O4Pb8] that involve edge-sharing O(1)Pb4 tetrahedra. The chains extend along [010] and are arranged into layers parallel to (100). The layers of OPb4 tetrahedra alternate with layers that contain VO4 tetrahedra as well as Pb(1), Pb(3), and Pb(4) atoms. The structural formula of the compound is Pb2[Pb2O](VO4)2, which emphasizes the Pb atoms that are not bonded to the additional Oadd atoms.