Infrared absorption of RbCl:Ag+and RbBr:Ag+: A theoretical treatment

A general model for $〈110〉$ off-center substitutional impurities in alkali-halide crystals is developed. It assumes a defect with a different mass from the ion it replaces and also with formal core-core short-range forces between the defect and its nearest neighbors different from those in the pure crystal. An absorption spectrum was calculated and three parameters, corresponding to the changes of the spring constants between the impurity and its nearest neighbors, were adjusted in order to fit absorption-band positions observed by Nolt and by Kirby, Hughes, and Sievers for RbCl: ${\mathrm{Ag}}^{+}$ at low frequencies. In a following paper this model has been used in conjunction with a very simple one-parameter-model version of lattice anharmonicity to predict a Raman scattering spectrum for RbCl: ${\mathrm{Ag}}^{+}$.