Nuclear Magnetic Relaxation in Ionic Crystals at High Temperatures

The spin-lattice relaxation time is measured by the recovery method for Na²³ nuclei and for F¹⁹ nuclei in NaCl, NaF, BaF₂, and LiF, at temperatures up to approximately 900^∘{}K. The Na²³ relaxation time is well predicted by phonon-phonon interaction theory at the lower temperatures, and by vacancy diffusion theory in higher ranges. The F¹⁹ 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 BaF₂. The samples are commercially produced single crystals of optical grade.