Electric Dipole Interactions Among Polar Defects in Alkali Halides

The influence of electric dipole interactions among substitutional polar molecules and atoms in the alkali halides is studied in the systems KCl: Li, KCl: OH, RbCl: OH, NaBr: F, and KCl: CN. A maximum is observed in the dc dielectric constant at a temperature $T_{max}$ which is shown to be proportional to the average interaction energy. It is also shown that this is not a relaxation effect. A remanent polarization is observed at low temperatures, which is caused by parallel-aligned pairs of dipoles experiencing a reorientation barrier. Specific-heat and thermal-conductivity measurements are also analyzed for the influence of interactions. Owing to the finite zero-field tunnel splitting of the impurity states, there is a threshold concentration, below which interaction effects are vanishingly small. The experimental results are compared to recent theoretical investigations and to results obtained on dilute magnetic alloys. In KCl: OH, the specific heat at high O${\mathrm{H}}^{-}$ concentrations $N$ is proportional to $N^{-\frac{1}{2}}{T}^{\frac{3}{2}}$. An unsuccessful search for direct evidence of polarization waves analogous to spin waves is reported and they are presumed heavily damped. It is conjectured that the above specific-heat result may be due to these modes.