Ionic Transfer in Tysonite-like Phases

The anion vacancy mechanism of conductivity for RE metal fluorides was confirmed by means of heterovalent doping of Lap3. A sharp increase of conductivity has been observed by doping LaF3 by MF and MF2 fluorides but its doping by TI1F4 leads to a decrease of the conductivity [97]. The vacancy mechanism of conductivity was also confirmed by F NMR [98]. 

Studying the mechanism of charge transfer in tysonite-like structures researchers combine sometimes F2 and F3 positions together and consider only two fluoride 

The results of F NMR studies of Lai xSrxF3 x (modification II) [102] show that the increase of x from 0 to 0.16 (i.e. the increase of Sr content and correspondingly of the anion vacancy concentration) leads to the decrease of the ratio of intensities of peaks corresponding to FI and F2,3 from 2.00 ( 0.05) to 1.80 ( 0.05). Thus in Lai xSrxF3 x solid solutions, vacancies occupy mainly FI positions. 

Copyright (1999) Springer. variable-temperature MAS NMR spectra of LaF, collected with a spinning speed of23 kHz ( — spinningsideband). Reprinted with permission from [99] (b) Copyright (1997) American Chemical Society.) 

The distances Fl-Fl and Fl-F2,3 are approximately equal but NMR studies showed that the fluoride ion lifetime in positions F2,3 is longer. It could be probably attributed to a lower free energy of these positions (higher coordinations with regard to La ) as well as to the lower number of possible jumps to nearest neighbour positions [111]. Thus, F-sites on B-planes act similarly to trap sites and F3 sites block the charge transport along c-direction. 

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