A¹⁹F NMR study of the anion mobility in lanthanum oxide fluorides

Conclusion

All the oxide fluorides studied show intensive diffusion of the fluoride ions, and this is responsible for their high electrical conductivity. The anisotropy of the magnetic shielding of the fluorine nuclei in the “rigid” lattice has been determined from the field dependence of the second moment. In the high-temperature range, the anisotropy was measured from the form of the resonance line. The coincidence of the values of the anisotropy at high and low temperatures shows that in stoichiometric lanthanum oxide fluoride below the temperature of the phase transformation into the cubic modification, the fluoride ions move only through the equivalent positions of the fluorine sublattice. Analysis of the chemical shifts and their anisotropy shows that the movement of the fluoride ions has vacancy character. Above T_c, the exchange of positions with oxygen is possible. The behavior of the spectra in the intermediate range indicates high dynamic nonuniformity of the fluoride ions, brought about by the existence of regions with an increased fluorine mobility, which apparently have a structure similar to that of the nonstoichiometric lanthanum oxide fluorides with an increased fluorine concentration. The replacement of oxygen by fluorine at first leads to a sharp increase in the mobility of the fluoride ions (x=0.04), after which it has practically no influence up to x=0.143. It is not excluded that the increase in the mobility is related not to the excess of fluorine but to a rearrangement of the crystal structure. Judging from the behavior of the spectral lines in the intermediate range, nonstoichiometric lanthanum oxide fluorides are dynamically more uniform.