Nuclear magnetic resonance study of superionic conductors with tysonite structure

The fluorine mobility in single crystals of the anion-deficient solid solution La 1-x Sr x F 3-x (x=0, 0.001, 0.003, 0.01, 0.03, 0.05 or 0.16) with the tysonite structure has been studied in the temperature range of 130-520 K by 19 F NMR. Different modes of ionic motion were identified and the leading mechanism of ionic diffusion has been suggested. The influences of temperature and fluorine vacancy concentration on the ionic mobility in tysonite have been investigated. In all crystals, fluorine diffusion begins in the F 1 sublattice. On increase in the temperature, ionic exchange between the F 1 and F 2,3 sublattices becomes observable. Addition of small amounts of Sr (x<0.003) significantly increases the ionic mobility between the sublattices. For 0.003<x<0.03 a further increase in x has no significant influence on the ionic exchange. Around x approximately=0.01 the beginning of cluster formation is noticeable. At higher concentrations (0.03<x<0.16) it is seen that ionic mobility decreases again and the guest-induced vacancies are preferentially located in the F 1 sublattice. At these doping concentrations the influence of large clusters seems to play a significant role.