DIFFUSION AND SHORT-RANGE ORDER WITHIN THE BA1-XBIXF2+X (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.45) SOLID-SOLUTION QUENCHED FROM 700-DEGREES-C

An investigation of the fluorite-type Ba1-xBixF2+x (0 ≤ x ≤ 0.45) solid solution has been carried out by conductivity measurements and by 19F NMR on samples quenched from 700°C. Application of the "clustering process" model to Ba1-xBixF2+x, which involves a conductivity maximum for xmax. ≅ 0.35, has allowed us to propose a progressive transformation within the solid solution of 4:4:3 clusters into 8:12:1 clusters as x increases. Interpretation of the NMR results is based on the existence of different fluoride ion sublattices and partial exchange between them at increasing temperature. A diffusive character of the F- ion motion is ascertained above T {reversed tilde equals} 320 K by comparing the activation energies determined by NMR and conductivity measurements. The NMR study confirms the validity of the clustering process proposed for Ba1-xBixF2+x and illustrates the difference in the electrical behavior of interstitial fluoride ions belonging to the 4:4:3 and 8:21:1 clusters. © 1992.