Experimental and Theoretical Study of the Far-Infrared Spectra of Monovalent Impurities in Sodium Chloride

Low-temperature measurements have been made of the far-infrared absorption due to a number of substitutional monovalent impurities in sodium chloride. The natural ${\mathrm{Cl}}^{35}$ and ${\mathrm{Cl}}^{37}$ isotopes, acting as simple mass defects, result in absorption in the acoustic band and yield information about the phonon frequencies in this region. This method has been utilized to improve Caldwell and Klein's shell-model calculation, and the results have been used to interpret the chemical impurity-induced absorption due to ${\mathrm{Ag}}^{+}$, ${\mathrm{Li}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{F}}^{-}$, ${\mathrm{Br}}^{-}$, and ${\mathrm{I}}^{-}$ ions in sodium chloride. Various defect models have been employed, involving a change of mass at the defect site together with changes in the force constants coupling the defect and its immediate neighbors. A simple nearest-neighbor central-force model has been found inadequate to explain the far-infrared spectra over a broad frequency range, and for the potassium and iodine impurities a model involving changes in the Szigeti effective charge of the defect has been used. The various defect models are compared and discussed, and possible improvements outlined.