Dipole-Dipole Interactions in Alkali Halides Doped with Hydroxyl.

The dielectric constant (epsilon) = (epsilon)' - i(epsilon)'' of KCl, NaCl, and KBr doped with OH('-) has been measured within the temperature range 0.030-200 K, the frequency range 10('2)-10('5) Hz, and the concentration range 80-4000 ppm at. Discrete features appear in both (epsilon)' and (epsilon)'' which are related to a thermally -activated two-step reorientation of OH('-) pairs of near -neighbor OH('-) ions. For OH('-) pairs other than the nearest-neighbor pair, this appears to be a two-phonon process, with each ion of the pair making one 90(DEGREES) step. No isotope effects were observed in the dielectric response of KCl when OH('-) was replaced by OD('-). A simple computation assuming an electric dipole-dipole interaction within the pair provides reasonable agreement with the measured temperature and frequency dependence. The amplitudes and number of peaks predicted by the model agree well with the data. The main difficulty in computing the pair interaction is the uncertain local-field correction for electric (and elastic) dipoles separated by a distance of order the lattice parameter. A subtle change in dielectric constant associated with a low-temperature phase transition has not been observed in KCl:OH, but a behavior similar to that found in glasses has been observed in KBr:OH at very low temperatures.