Far-Infrared Properties of Lattice Resonant Modes. IV. Paraelectric Impurities

We have studied the three paraelectric defect systems KCl: ${\mathrm{Li}}^{+}$, RbCl: ${\mathrm{Ag}}^{+}$, and NaCl: O${\mathrm{H}}^{-}$ through the temperature dependence and electric field dependence of their far-infrared absorption spectra. It is found that the motional energy states for the defects in these three systems have many properties in common. In each case the impurity ion has an associated electric dipole moment and low-lying tunneling energy levels. The defects also have higher energy levels which can be attributed, at least in part, to c. m. resonant-mode motion. The KCl: ${\mathrm{Li}}^{+}$ tunneling levels are found to be well described by the model of Gomez, Bowen, and Krumhansl, assuming a ${\mathrm{Li}}^{+}$ dipole moment of 1.14 ± 0.05 $e$Å for both ${\mathrm{Li}}^6$ and ${\mathrm{Li}}^7$. The most surprising feature of the KCl: ${\mathrm{Li}}^{+}$ higher-lying energy levels is that they show a negative isotope shift [i.e., $\overline{\omega }\left({\mathrm{Li}}^6\right)<\mathrm{}\overline{\omega }\left({\mathrm{Li}}^7\right)$]. The three observed absorption lines in RbCl: ${\mathrm{Ag}}^{+}$, as well as their dependence on an external electric field, are shown to be consistent with a [110] off-center position for the ${\mathrm{Ag}}^{+}$ ions. The far-infrared absorption spectrum of NaCl: O${\mathrm{H}}^{-}$ is complicated, with at least eight absorption lines between 2 and 22 ${\mathrm{cm}}^{-1\mathrm{}}$. Considerable agreement is found between the experimental results and a model which incorporates both librational tunneling of the O${\mathrm{H}}^{-}$ ion and c.m. vibrational motion.