Structures of Bi₁₄WO₂₄ and Bi₁₄MoO₂₄ from neutron powder diffraction data

The (isomorphous) structures of Bi₁₄WO₂₄, tetradecabismuth tungsten tetracosaoxide, and Bi₁₄MoO₂₄, tetradecabismuth molybdenum tetracosaoxide, have been solved and refined using neutron powder diffraction data in the space group I4/m. The metal-atom array is fully ordered in terms of composition, and in terms of atomic positions deviates only slightly from a fluorite-type δ-Bi₂O₃-related parent structure. Three independent O-atom sites (accounting for 70 out of 78 O atoms in the unit cell) are also very close to fluorite-type parent positions. The remaining two O-atom sites, which coordinate W, exhibit partial occupancies and displacive disorder, neither of which could be better modelled by lowering of symmetry. The W site is coordinated by four O atoms in highly distorted tetrahedral coordination, the tetrahedron necessarily being orientationally disordered on that site. Nonetheless, the structure appears to be chemically reasonable.