Nuclear Quadrupole Spin-Lattice Relaxation in Alkali Halides

Abstract

Nuclear quadrupole spin-lattice relaxation times have been measured in alkali halide crystals by the pulsed magnetic resonance technique. Measurements were made on ${\mathrm{Na}}^{23}$ in NaCl, NaBr, and NaI; ${\mathrm{Cl}}^{35}$ in NaCl and KCl; ${\mathrm{Br}}^{79,81}$ in NaBr, KBr, RbBr, and CsBr; ${\mathrm{Rb}}^{87}$ in RbCl and RbBr; and ${\mathrm{I}}^{127}$ in NaI, KI, and CsI. Over a temperature range of 298°K to 195°K the relaxation times are inversely proportional to the square of the absolute temperature. The data are compared to relaxation times calculated from an ionic crystal model of Van Kranendonk and a covalent model of Yosida and Moriya. The ionic model is modified to include the interaction between the nuclear quadrupole moment and the electric field gradient due to electric dipole moments associated with optical modes of vibration. Neither of these models alone predicts the experimental relaxation times for all cases, but a combination of the two effects is required. The modified ionic model applies reasonably well to crystals which contain the lighter ions.