Synthesis and Crystal Structure of Bi6.4Pb0.6P2O15.2. A New Polymorph in the Series Bi6+xM1-xP2O15+y.

Abstract Bi 6.4 Pb 0.6 P 2 O 15.2 is a polymorph of structures with the general stoichiometry Bi 6+ x M 1− x P 2 O 15+ y . However, unlike previously published structures that consist of layers formed by edge sharing OBi 4 tetrahedra bridged by PO 4 and TO 6 ( T =transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C 2, a =19.4698(4) A, b =11.3692(3) A, c =16.3809(5) A, β =101.167(1)°, Z =10. Single-crystal X-ray diffraction data were refined by least squares on F 2 converging to R 1 =0.0387, w R 2 =0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb) 4 units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb–O bond lengths vary from 2.265(14) to 2.869(14) A. Pairs of such cubes share an edge to form a Pb 3 O 20 unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb) 4 units, and some oxygen ions of the polyhedra are also part of PO 4 tetrahedra. One, two, three and or four PO 4 moieties are connected to the Bi polyhedra. Bi–O bond lengths ⩽3.1 A vary from 2.090(12) to 3.07(3) A. The articulations of Pb cubes, Bi polyhedra and PO 4 tetrahedra link into the three-dimensional structure.