Paraelectric Resonance of KCl:Li+

Paraelectric resonance spectra of KCl:${\mathrm{Li}}^{7+}$ and KCl:${\mathrm{Li}}^{6+}$ are presented for three different frequencies: 63, 24, and 9 GHz. Quantitative results for the case of static electric field ${\mathbf{E}}_0$∥[001] support the off-center tunneling model due to Gomez, Bowen, and Krumhansl. We obtain these values for the dipole moment $\mu$ and zero-field splitting $\delta$: ${\mu }_7={\mu }_6=5.6\mathrm{}\pm 0.2$ D, ${\delta }_7=0.77\mathrm{}\pm 0.03$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\delta }_6=1.07\mathrm{}\pm 0.04$ ${\mathrm{cm}}^{-1\mathrm{}}$. These agree with results from several other experimental techniques. Detailed analysis of line shapes and intensities indicates the presence of large random internal strains and of random electric fields. A satisfactory explanation for the presence of these is still lacking. The 63-GHz spectrum for ${\mathbf{E}}_0$∥[011] shows a negative resonance ("hole") similar to a feature seen previously in ESR experiments on MgO:${\mathrm{Ni}}^{2+}$ and MgO:${\mathrm{Fe}}^{2+}$. The model of Gomez et al. is shown to satisfy requirements for the occurrence of this negative resonance.