Structures of Bi14WO24 and Bi14MoO24 from neutron powder diffraction data

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