Study of Iron Ions in NaCl Using the Mössbauer Effect. II. The Vacancy-Impurity Associated State

Two types of Mössbauer spectra were observed for dilute concentrations of ${\mathrm{Fe}}^{57}$ ions in NaCl. A doublet with a 0.5 mm/sec quadrupole splitting and a 0.5 mm/sec isomer shift relative to stainless steel (80°K) is interpreted as the result of either ${\mathrm{Fe}}^{2+}$ or ${\mathrm{Fe}}^{3+}$ associated with a positive ion vacancy. A broad unsplit line at -2.07±0.07 mm/sec (80°K) is interpreted as the result of ${\mathrm{Fe}}^{+}$ substitutionally replacing ${\mathrm{Na}}^{+}$. The data are used to make a rough estimate of the binding energy between positive ion vacancies and cobalt impurities, and the result is consistent with theoretical expectations. A study of the temperature dependence of these two resonances indicates that the ratio of the recoilless emitted $\gamma$ rays from the isolated ions compared with the vacancy associated ions is 0.75±0.15 at 80°K, 0.24±0.08 at 296°K, and 0.16±0.08 at 478°K. This huge effect is believed to be due to the difference in elastic constants and "mass-symmetry" contributions to the localized modes, for the two environments.