Ionic conduction mechanisms in the Ba 1−xBi xF 2+x solid solution quenched from 700°C : conductivity, NMR and neutron diffraction

Abstract

An investigation of the Ba 1−xBi xF 2+x (0 ⩽ x ⩽ 0.45) solid solution of CaF 2-type structure has been carried out by conductivity measurements, neutron diffraction and 19F NMR on samples quenched from 700°C . The application of the “clustering processes” model [1] to Ba 1−xBi xF 2+x which involves a conductivity maximum for xmax. # 0.35 has allowed to propose within that solid solution the progressive transformation of 4:4:3 clusters into 8:12:1 cubooctahedral clusters when x increases . The clustering process proposed in Ba 1−xBi xF 2+x is in agreement with the distribution of the fluoride ions in normal and interstitial sites determined by neutron diffraction. The NMR results are interpreted on hand of the existence of different fluoride sublattices and partial exchanges between them at rising temperature . The diffusive character of the mobile F − ions motion is shown above T # 320 K . The NNR study confirms so the validity of clustering process proposed in Ba 1−xBi xF 2+x and shows the unlike electrical behavior of interstitial fluoride ions belonging to 4:4:3 and 8:12:1 clusters.