Crystal structures of Pb8O5(AsO4)(2) and Pb5O4(CrO4), and review of PbO-related structural units in inorganic compounds

摘要

The crystal structures of Pb8O5(AsO4)(2) and Pb5O4(CrO4) have been structurally characterized using crystals prepared by high-temperature solid-state reactions. The structure of Pb8O5(AsO4)(2) (monoclinic, C2/m, a = 10.797(2), b = 10.429(2), c = 14.614(3)Angstrom, beta = 98.081(3)degrees, V = 1629.3(6) Angstrom(3)) has been refined to R-1 = 0.034 on the basis of 1445 unique observed reflections. Pb8O5(AsO4)(2) is isotypic to PbsO(5)(PO4)(2). It is based upon complex [O5Pb8] sheets consisting of OPb4 tetrahedra and strongly distorted OPb6 octahedra. These sheets are parallel to (001) and are linked via AsO4 groups located in the interlayer regions. The structure of Pb5O4(CrO4) (monoclinic, P2(1)/c, a = 14.792(2) b = 11.677(2), c = 11.582(2)Angstrom, beta = 90.979(3)degrees, V = 2000.3(5) Angstrom(3)) has been refined to R-1 = 0.051 on the basis of 1915 unique observed reflections. It is isotypic to Pb5O4(MoO4) but not to Pb5O4(SO4). The structure is based upon [O3Pb5] chains of edge-sharing OPb4 tetrahedra extended along [100] and arranged parallely within the (001) plane. The adjacent [O3Pb5] chains are linked via OPb3 triangular groups of atoms along [001]. The CrO4 tetrahedra are located between the [O3Pb5] chains. The [O4Pb7] sheet and [O3Pb5] chain observed in the structures may be considered as derivatives of the continuous [OPb] sheet of OPb4 tetrahedra from the tetragonal modification of PbO. Both units can be obtained from the [OPb] sheet by a removal of a certain portion of OPb4 tetrahedra. This may be modelled using a lattice representation of tetrahedral sheets, where black and white squares correspond to OPb4 tetrahedra and vacancies, respectively. The review of PbO-related units (eleven layers and eleven chains; consisting of both anion- and cation-centered tetrahedra) in inorganic compounds is provided with more than 80 compounds taken into account. (C) 2003 Elsevier Inc. All rights reserved.