High temperature NMR study of the local structure of molten LaF3-AF (A = Li, Na, K and Rb) mixtures.

The local structures of molten lanthanum alkali fluoride binaries have been studied using HT NMR technique. The chemical shifts of (19)F, (23)Na and (139)La in solid and in liquid have been compared for AF (A = alkali) and LaF(3). In pure molten alkali fluorides, the polarisability of anion-cation pairs appears to be a key parameter to depict the observed evolution of (19)F chemical shifts. The influence of the composition has also been studied by measuring the chemical shifts in molten LaF(3)-AF as a function of LaF(3) concentration. A strong influence of the alkali influence is observed. The coordination number of lanthanum is decreased versus AF amount all the more since the alkali atomic number is high. Moreover, the more polarisable the alkali, the less bridging fluorines between the LaF(x) units.