Modelling, refinement and analysis of the 'Type III' δ-Bi2O3-related superstructure in the Bi2O3-Nb2O5 system

The structure of the 'Type III' δ-Bi2O3-related superstructure phase in the system Bi2O3-Nb2O5 is presented. A starting model was constructed by considering the crystal-chemistry of the system in the context of symmetry constraints determined by electron diffraction. After applying initial distortions, this could be Rietveld-refined against a combination of synchrotron X-ray and time-of-flight neutron powder diffraction data. The undistorted starting model was independently optimized using solid-state ab initio energy calculations, giving a fully optimized structure in excellent agreement with that obtained by Rietveld refinement. This dual approach both validates the structure and demonstrates the value of combining accurate total energy calculations with traditional refinement techniques for the solution of complex structures using powder diffraction data. The structure (Bi94Nb32O221, Z=1, I4-barm2 (no. 119), a=11.52156(18), c=38.5603(6) A) consists of interacting corner-connected strings of NbO6 octahedra along <110>F directions of the FCC subcell, and can be described as a hybrid of fluorite and pyrochlore types