Na Nuclear Magnetic Relaxation of NaCl Crystals Containing Lattice Imperfections

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Nuclear magnetic resonance of Na nuclei was measured in NaCl crystals containing divalent impurity ions such as Ca⁺⁺, Cd⁺⁺, Mn⁺⁺ and Sr⁺⁺, at 14 Mc/sec and in the temperature range between 10°C and 650°C. The spin-lattice relaxation at temperature above ∼400°K was found to be caused by the electric quadrupole interaction of Na nucleus modulated by the lattice diffusion of Na-ion-vacancies, and a minimum in T₁ occured at about 588°K for all samples where the jumping frequency of the vacancy is equal to about 1.66 times the Larmor frequency. Some difference was observed in the temperature dependence of T₁ for Ca⁺⁺ and Cd⁺⁺ doped crystals and was discussed in connection with the complex diffusion. By analysing the temperature dependence of T₁ for crystals containing Ca⁺⁺ impurities with the use of a modified Torrey’s equation, the activation energy for a vacancy jump is obtained to be 0.74 eV. In the case of Cd⁺⁺ impurities, the association energy of the complex formation was about 0.4 eV in accordance with the result of the ionc conductivity experiments. Further, the temperature dependence of T₁ above 600°K is discussed in connection with the diffusion of the divalent ion. Also the temperature dependence of the spin-spin relaxation time T₂ was measured, and it was found that the diffusion of Na ion via the vacancy jump is responsible to the motional narrowing of the line width.