The present work is a study by nmr pulse techniques of the motion of fluorine atoms in lanthanum trifluoride as a function of temperature, between 100 and 560°C. The experiments have been conducted with a single crystal of La${\mathrm{F}}_3$. The model for the motions derived from this study is the following: There are two types of fluorine nuclei, the spins $I$ and the spins $S$, located on different sublattices. Between 100 and 300°C, the motion of the spins $I$ is fast (i.e., such as to be appreciable in times shorter than the reciprocal of the rigid-lattice line width), and the motion of the spins $S$ is slow. There is an exchange of atoms between the two sublattices, the rate of which is slow up to about 300°C and fast at higher temperature. The ratio of populations of the spins $I$ and $S$ is $\frac{N_I}{N_S}=2\mathrm{}$. Approximate values are derived for the activiation energies associated with these two types of motion.

• Received 29 November 1965

DOI:https://doi.org/10.1103/PhysRev.144.321