Crystal structures of Pb 8O 5(AsO 4) 2 and Pb 5O 4(CrO 4), and review of PbO-related structural units in inorganic compounds
The crystal structures of Pb 8O 5(AsO 4) 2 and Pb 5O 4(CrO 4) have been structurally characterized using crystals prepared by high-temperature solid-state reactions. The structure of Pb 8O 5(AsO 4) 2 (monoclinic, C2/ m, a=10.797(2), b=10.429(2), c=14.614(3)Å, β=98.081(3)°, V=1629.3(6) Å 3) has been refined to R1=0.034 on the basis of 1445 unique observed reflections. Pb 8O 5(AsO 4) 2 is isotypic to Pb 8O 5(PO 4) 2. It is based upon complex [O 5Pb 8] sheets consisting of OPb 4 tetrahedra and strongly distorted OPb 6 octahedra. These sheets are parallel to (001) and are linked via AsO 4 groups located in the interlayer regions. The structure of Pb 5O 4(CrO 4) (monoclinic, P2 1/ c, a=14.792(2), b=11.677(2), c=11.582(2)Å, β=90.979(3)°, V=2000.3(5) Å 3) has been refined to R1=0.051 on the basis of 1915 unique observed reflections. It is isotypic to Pb 5O 4(MoO 4) but not to Pb 5O 4(SO 4). The structure is based upon [O 3Pb 5] chains of edge-sharing OPb 4 tetrahedra extended along  and arranged parallely within the (001) plane. The adjacent [O 3Pb 5] chains are linked via OPb 3 triangular groups of atoms along . The CrO 4 tetrahedra are located between the [O 3Pb 5] chains. The [O 4Pb 7] sheet and [O 3Pb 5] chain observed in the structures may be considered as derivatives of the continuous [OPb] sheet of OPb 4 tetrahedra from the tetragonal modification of PbO. Both units can be obtained from the [OPb] sheet by a removal of a certain portion of OPb 4 tetrahedra. This may be modelled using a lattice representation of tetrahedral sheets, where black and white squares correspond to OPb 4 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.