Nuclear Spin-Lattice Relaxation in Alkali Halides at Low Temperatures

The spin-lattice relaxation time $T_1$ has been measured for the ${\mathrm{Br}}^{79}$ and ${\mathrm{Br}}^{81}$ species in NaBr and LiBr and for the ${\mathrm{I}}^{127}$ species in NaI and RbI in the temperature range 15-77.3°K and also at room temperature, using the 180°-90° pulse technique. The data were fitted to the Van Kranendonk theory using several theoretical phonon spectra. The Van Kranendonk theory satisfactorily accounts for the gross temperature dependence of $T_1$ over the entire range investigated. A consistent deviation of $T_1$ to larger values than those predicted by the theory was noted in all of the samples in the range $\simeq 30-60$° K. The theory is unable to account for these deviations in detail, but the results indicate that optical-phonon relaxation is important even at these low temperatures.