Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi6.4Pb0.6P2O15.2 is a polymorph of structures with the general stoichiometry Bi6+xM1−xP2O15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi4 tetrahedra bridged by PO4 and TO6 (T=transition metal) tetrahedra and octahedra the title compoundʹs structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F2 converging to R1=0.0387, wR2=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) Å. Pairs of such cubes share an edge to form a Pb3O20 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 PO4 tetrahedra. One, two, three and or four PO4 moieties are connected to the Bi polyhedra. Bi–O bond lengths ⩽3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO4 tetrahedra link into the three-dimensional structure.