The defect structure of the title compound has been analyzed by reverse Monte Carlo (RMC) modeling of neutron total scattering data. The composition exhibits both diffuse scattering and weak superlattice ordering of the cubic fluorite subcell in neutron diffraction patterns. Combined Rietveld analysis of X-ray and neutron data at room temperature reveals oxide ion scattering on three crystallographic sites. Analysis of the RMC model reveals Bi coordination numbers consistent with stereochemical activity of the Bi 6 s(2) lone pair electrons. Integration of the O-M-O angular distribution function gives an angular ratio consistent with predominantly < 110 > vacancy ordering in this system. (C) 2010 Elsevier B.V. All rights reserved.