Far Infrared Properties of Lattice Resonant Modes. I. Isotope Shifts

The frequency shift of a lattice resonant mode associated with the isotopic substitution of an impurity ion has been measured for NaCl: Cu⁺, KI: Ag⁺, and KBr: Li⁺. For all three systems, the experimental frequency shifts can be described satisfactorily by an Einstein oscillator representation for the resonant mode with the oscillator mass set equal to the impurity-ion mass. The model of Benedek and Nardelli and also Klein does not account for the experimental results of NaCl: Cu⁺ or KBr: Li⁺. The inclusion of anharmonic forces does improve the agreement between theory and experiment, but a large discrepancy still exists for KBr: Li⁺.