An NMR study of molecular reorientations and diffusion in solid LiBF4

摘要

The19F second moment and the19F,11B, and7Li spinhyphen;lattice relaxation times in a powdered sample of LiBF4have been measured as a function of temperature. The results show that the BFminus;4ion reorients isotropically while the Li+ion remains stationary. The activation energy associated with the BFminus;4motion is 8.5 kcal/mol. Above sim;385 K the BFminus;4and Li+diffuse through the lattice resulting in a19F second moment which is almost zero. The activation energy of this process is 19.2 kcal/mol. A librational motion presumably influences the19FT1rgr;results just before diffusion sets in. The migration of Li+around the BFminus;4ion, which seems to be a strong possibility in the case of an isolated LiBF4molecule, does not take place in the solid.