Anelastic and Dielectric Relaxation due to Impurity-Vacancy Complexes in NaCl Crystals

Appreciable pairing of divalent metallic impurities with ${\mathrm{Na}}^{+}$ vacancies occurs in NaCl below 300°C. The reorientation of such pairs or complexes had previously been observed under an applied electric field. In the present work stress-induced reorientation of pairs in NaCl doped with Ca${\mathrm{Cl}}_2$ and Mn${\mathrm{Cl}}_2$ has been studied by means of internal friction measurements. An internal friction peak attributed to pair reorientation under stress was observed near 100°C for a vibration frequency of ∼10kc/sec. Data obtained for longitudinal stress along both the $〈100〉$ and $〈111〉$ crystal directions yields information about the rates of relaxation corresponding to various mechanical relaxational modes. The data can be interpreted consistently in terms of an extension of the theory previously applied to dielectric relaxation, according to which the paired vacancy occupies only nearest-neighbor (n.n.) and next-nearest-neighbor (n.n.n.) sites to the impurity. Relations obtained between the relaxation rates and the various possible jump rates for a ${\mathrm{Na}}^{+}$ ion into the vacancy enable each of the specific vacancy jump rates to be determined. It is concluded that the most rapid means for the reorientation of an impurity-vacancy pair between two n.n. sites is for the vacancy to move via a n.n.n. site. The rate of jump of the impurity ion into the vacancy is found to be a relatively slow process.