Far-infrared properties of lattice resonant modes. VII. Excited states and paraelectric pairs

Far-infrared transmission measurements have been made on single-crystal KBr:${\mathrm{Li}}^{+}$ and NaCl:${\mathrm{Cu}}^{+}$ samples up to 16 cm in length. At 1.2 K, weak satellite lines are observed at frequencies around the strong lattice-resonant-mode transition. With increasing temperature the strengths of the resonant mode and the satellite lines decrease and excited-state transitions increase in strength. Although we assume that these temperature-activated lines are excited-state transitions to the overtone levels of the resonant mode, no model potential has been found which is completely consistent with all of the infrared and Raman data. We demonstrate experimentally that these excited-state transitions can be used to monitor the mean impurity concentration of these long, inhomogeneously doped crystals. Then, using this new technique, we show that most of the satellite modes with frequencies below the resonant-mode frequency are due to impurity pairs while weak modes with frequencies near the resonant-mode frequency are associated with isolated impurities. For KBr:${\mathrm{Li}}^{+}$, a 50%-50% isotopic mixture of the dopant is used to show that the low-frequency paraelectric modes reported by Bridges arise from a diatomic Li center. Whether the center is a ${\mathrm{Li}}_2$ ${\mathrm{}}^{+}$ ion or two ${\mathrm{Li}}^{+}$ ions in next-nearest-neighbor sites has not been resolved.