Synthesis and crystal structure of Bi 6.4Pb 0.6P 2O 15.2. A new polymorph in the series Bi 6+xM 1-xP 2O 15+y

Bi _6.4Pb _0.6P ₂O _15.2 is a polymorph of structures with the general stoichiometry Bi _6+xM _1-xP ₂O _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, 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, w R₂=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 Pb ₃O 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 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO ₄ tetrahedra link into the three-dimensional structure.