Far-Infrared Properties of Lattice Resonant Modes. II. Stress Effects

The resonant-mode frequency shifts produced by uniaxial stress have been measured for KBr: ${\mathrm{Li}}^{+}$ and KI: ${\mathrm{Ag}}^{+}$. Relative frequency shifts of up to 15% are observed for stresses of 3 kg/${\mathrm{mm}}^2$. For both defect systems, the observed splitting pattern is consistent with a defect center of cubic symmetry; moreover, the absence of low-lying tunneling states implies that the equilibrium position of the respective impurity ion is at the normal $K^{+}$ lattice site. The considerable difference between the measured stress coupling coefficients for the defects is taken as evidence that stress shifts of resonant modes reflect the bonding of the particular impurity ion with its neighbors.