Nuclear Magnetic Relaxation in LiF at High Temperatures

Abstract

Measurements of $T_1$ and $T_2$, the spin-lattice and spin-spin relaxation times, are reported for ${\mathrm{Li}}^7$ and ${\mathrm{F}}^{19}$ nuclei in a single crystal of LiF. $T_1\left(\mathrm{Li}\right)$ has been measured from room temperature to the melting point (1120°K) and the other quantities from 830°K to the melting point. Above 620°K, relaxation is caused exclusively by the diffusion of Li and F ions. $T_1\left(\mathrm{Li}\right)$, $T_2\left(\mathrm{Li}\right)$, and $T_1\left(\mathrm{F}\right)$ are mainly determined by ${\nu }_{\mathrm{Li}}$, the jump frequency of Li ions, and $T_2\left(\mathrm{F}\right)$ mainly by ${\nu }_{\mathrm{F}}$. Using a theory of relaxation developed elsewhere, ${\nu }_{\mathrm{Li}}$ and ${\nu }_{\mathrm{F}}$ are obtained for the region of intrinsic diffusion as ${\nu }_{\mathrm{Li}}=1.7\mathrm{}\times {10}^{16}\exp (-1.81\mathrm{} \mathrm{eV}/kT)$ and ${\nu }_{\mathrm{F}}=4.5\mathrm{}\times {10}^{17}\exp (-2.2\mathrm{} \mathrm{eV}/kT)$. For the extrinsic region, the motional activation energy of Li ions is obtained as 0.71 eV. The values of ${\nu }_{\mathrm{Li}}$ are in good agreement with those obtained from conductivity measurements; to our knowledge, ${\nu }_{\mathrm{F}}$ has never been measured by other methods. Below 620°K quadrupolar relaxation due to lattice vibrations and relaxation by paramagnetic impurities become important, and rough values are obtained for these contributions to $T_1\left(\mathrm{Li}\right)$. Large angular variations of $T_1$ and $T_2$ are observed in the region of relaxation due to atomic diffusion. $T_2\left(\mathrm{Li}\right)$ and $T_1\left(\mathrm{Li}\right)$ vary by a factor of 2 and 1.5, respectively, as a function of crystal orientation, in good agreement with theoretical prediction. A crude measurement of $T_1\left(\mathrm{Li}\right)$ in molten LiF is discussed.