Nuclear Magnetic Relaxation in Ionic Crystals at High Temperatures

Abstract

The spin-lattice relaxation time is measured by the recovery method for ${\mathrm{Na}}^{23}$ nuclei and for ${\mathrm{F}}^{19}$ nuclei in NaCl, NaF, Ba${\mathrm{F}}_2$, and LiF, at temperatures up to approximately 900°K. The ${\mathrm{Na}}^{23}$ relaxation time is well predicted by phonon-phonon interaction theory at the lower temperatures, and by vacancy diffusion theory in higher ranges. The ${\mathrm{F}}^{19}$ relaxation time at the lower temperatures (below 500°K-700°K, depending on the compound) is attributed to spin-spin diffusion in conjunction with paramagnetic impurities; at higher temperatures a decrease consistent with the effects of vacancy diffusion is observed, with an activation energy of 0.65 ev in NaF and of 0.82 ev in Ba${\mathrm{F}}_2$. The samples are commercially produced single crystals of optical grade.