A new structural type built up on the association of [Bi2O2]2+ infinite layers surrounded by PO4 and M/Bi(O,F)6 octahedra was reported. This form was a new polymorph corresponding to the general formula Bi6(M/Bi)P2O16−x, different from the so-called polytype 1 by a shearing of [(M/Bi)P2O16−x]2− slabs sandwiched between [Bi2O2]2+ layers. In these series of compounds, the octahedral Mn+ cation was in strong competition with penta-coordinated Bi3+ accompanied by O vacancies. Here, the rational substitution of F− for O2− was used to avoid the M/Bi statistic distribution in octahedral sites independent of the valency of the M cation. Both monocrystalline and powder X-ray diffraction and electron diffraction were used to distinguish and rationalize the adopted polymorphic form as a function of M. This distinction was particularly fine since X-ray diffraction powder patterns of polytypes 1 and 2 were nearly identical, due to the unchanged [Bi2O2]2+ based sub-lattice.

Polymorphism and Anionic Vacancies in the Bi6(M,Bi)1P2(O,F)16−x Aurivillius Derivatives. M.Colmont, M.Huvé, E.M.Ketatni, O.Mentré: Solid State Sciences, 2008, 10[5], 533-43