Probing the local structure of Bi ₂ O ₃ chemical derivatives: the neglected cation sublattice

High-resolution synchrotron X-ray diffraction data confirm the stabilization of the average structure at ambient temperatures as a defect fluorite structure in ternary and quaternary oxides of Bi₂O₃, where some Bi cations are replaced with Dy³⁺ and/or Er³⁺, Nb⁵⁺ and W⁶⁺. Rietveld refinement of the diffraction data indicates that when the larger cation Er³⁺ (0.89 Å) is replaced with smaller cations W⁶⁺ (0.42 Å) and/or Nb⁵⁺ (0.48 Å), the unit-cell parameter a, based on the relative ionic radii of the substituents, counterintuitively increases. Total scattering data show that the identity of the cation substituent is important in determining the positions of some Bi cations in the defect fluorite structure. Er³⁺ and Dy³⁺ cations induce monoclinic-like distortions that extend only locally in the Bi³⁺ sublattice. X-ray absorption spectroscopy provides evidence that some Dy cations, in the δ phases, prefer to keep a similar local environment to that in the parent Dy₂O₃. W cations are found to be predominantly tetrahedrally coordinated in the δ phases explored in this work.