Dependence of Nuclear Quadrupole Relaxation Times in Ionic Crystals on the Phonon Spectrum

Nuclear quadrupole relaxation times are calculated for the halogen and alkali nuclei in a number of alkali halides using lattice phonon density curves computed by Karo. The calculated values of $T_1$ at room temperature are generally lower than those obtained using the Debye model and in better agreement with experiment. The variation of $T_1$ with temperature has also been calculated over the range 20°K to room temperature for ${\mathrm{Na}}^{23}$ and ${\mathrm{I}}^{127}$ nuclei in NaI. Excellent agreement is obtained with the experimental data on the temperature variation of $T_1$ when use is made of the Karo distribution function. The Debye model gives a temperature variation comparable with experimental above 70°K but a relatively slower variation below 70°K.