Short-range order and diffusion processes in the Na1-xBixF1+2x anion-excess solid solution

The F− diffusion processes within the anion-excess solid solution Na1−xBixF1+2x with fluorite-type structure have been studied by 19F-NMR investigation. Two types of fluoride ions, Fn and F′, are distinguished in the 19F-NMR spectra of rigid lattice at low temperature (T=175 K) and the distribution of fluoride ions among the two sites is determined by deconvolution of the experimental spectra. The short-range order in these materials is analogous to the clustering process observed in the K1−xBixF1+2x solid solution. It corresponds to an order between cubic entities and cubooctaedral clusters. Two diffusion mechanisms are identified on both sides of a Tr temperature. The first one at TTr to an F− exchange between cubic and cubooctaedral clusters