Electron Spin Resonance ofMn++in Alkali Chlorides: Association with Vacancies and Impurities

The spin resonance of ${\mathrm{Mn}}^{++}$ in LiCl, NaCl, and KCl is described. In NaCl, five different spectra are observed. Designated I, II, ${\mathrm{III}}_1$, ${\mathrm{III}}_2$, and IV, these arise from ${\mathrm{Mn}}^{++}$ ions in five different environments. These are identified as (I) ${\mathrm{Mn}}^{++}$ ions in an aggregated or precipitated state, (II) isolated ${\mathrm{Mn}}^{++}$ ions which are not near any defect, (${\mathrm{III}}_1$) ${\mathrm{Mn}}^{++}$ ions with a positive ion vacancy bound in the nearest cation site, (${\mathrm{III}}_2$) ${\mathrm{Mn}}^{++}$ ions with a vacancy bound in the next-nearest site, and (IV) ${\mathrm{Mn}}^{++}$ ions paired off with a chemical impurity charge compensator. In LiCl and KCl only the two vacancy-${\mathrm{Mn}}^{++}$ pairs (${\mathrm{III}}_1$ and ${\mathrm{III}}_2$) are studied. In all three salts the nearest and next-nearest vacancy-${\mathrm{Mn}}^{++}$ pairs are approximately equally stable. By studying the spectra in NaCl as the temperature is raised, the thermal dissociation of the ${\mathrm{Mn}}^{++}$-vacancy pairs is observed, with a corresponding increase in the number of isolated ${\mathrm{Mn}}^{++}$ ions. This follows a simple mass action law giving a binding energy for the ${\mathrm{Mn}}^{++}$-vacancy pairs of approximately 0.4 ev. An ionic point charge model of the crystalline field produced by the defects is successful in explaining several features of the spectra. However, the magnitude of the field predicted in this manner appears to be a factor of ten too low if the recent theory of Watanabe is used. This discrepancy is attributed to the role of overlap of the ion cores and/or covalency.