Nuclear Spin-Lattice Relaxation in Alkali Halides at Low Temperatures

The spin-lattice relaxation time T₁ has been measured for the Br⁷⁹ and Br⁸¹ species in NaBr and LiBr and for the I¹²⁷ 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 T₁ over the entire range investigated. A consistent deviation of T₁ to larger values than those predicted by the theory was noted in all of the samples in the range 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.