Crystal structures and polymorphism in compounds Bi6+xT1−xP2O15+y, T=first row transition metals and Pb

The compounds Bi6+xT1−xP2O15+y, T=Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn and Pb display five polymorphic forms. Polymorph A is formed by the Ti, Mn, Fe and Ni phases. Polymorph B is exhibited by Co and Cu compounds. The Cr phase crystallizes as polymorphic form C and the Zn phase crystallizes as polymorph D. The Pb compound crystallizes in a new structure type designated as polymorph E. The transition metal crystal structures demonstrate a similar motive. OBi4 tetrahedra share edges to form two-dimensional Bi2O2 layers that are spanned by PO4 tetrahedra and TO6−y octahedra, pyramids and a trigonal bipyramid to form a three-dimensional network. Polymorph A crystallizes in space group C2; polymorph B is centrosymmetric with space group C2/c, the unit cell parameters differ and the unit cell volume is about double. Polymorph C crystallizes in space group image and polymorph D exhibits space group C2. Bi6.4Pb0.6P2O15.2 can be considered as polymorph E, space group C2, with a new crystal structure but related stoichiometry.