NUCLEAR-MAGNETIC-RESONANCE STUDY OF SUPERIONIC CONDUCTORS WITH TYSONITE STRUCTURE

The fluorine mobility in single crystals of the anion-deficient solid solution La1-xSrxF3-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 F-19 NMR. Different modes of ionic motion were identif ied and the leading mechanism of ionic diffusion has been suggested. T he influences of temperature and fluorine vacancy concentration on the ionic mobility in tysonite have been investigated. In all crystals, f luorine diffusion begins in the F-1 sublattice. On increase in the tem perature, ionic exchange between the F-1 and F-2,F-3 sublattices becom es observable. Addition of small amounts of Sr (x < 0.003) significant ly increases the ionic mobility between the sublattices. For 0.003 < x < 0.03 a further increase in x has no significant influence on the io nic exchange. Around x approximate to 0.01 the beginning of cluster fo rmation is noticeable. At higher concentrations (0.03 < x < 0.16) it i s seen that ionic mobility decreases again and the guest-induced vacan cies are preferentially located in the F-1 sublattice. At these doping concentrations the influence of large clusters seems to play a signif icant role.