Dielectric Relaxations in Additively Colored KCl and KCl: SrCl2Crystals

The dielectric relaxation of additively colored alkali halides has been investigated by means of thermocurrents (TC) and optical-absorption measurements. In addition to the TC bands present in noncolored KCl crystals, several new bands have been found when $F$ centers were partially converted to other centers such as $F^{\prime }$, $M$, $N$, $R$, $Z_1$, $Z_2$, and $Z_3$ by $F$-light irradiation at various temperatures. These additional TC bands were found to be due to the migration of electrons rather than to the orientation of ionic dipoles. These electrons may presumably be trapped at various sites in colored crystals and are thermally and optically liberated. In the particular case of the $Z_2$ centers, which can be obtained only in crystals containing metallic divalent impurities, the TC and the field-induced dicroism measurements did not provide any evidence for the existence of an electric dipole moment of the $Z_2$ center itself. The very intense dielectric losses measured in crystals containing $Z_2$ centers could be identified with the additional TC bands due to the migration of electrons. Quantitative measurements of the impurity-vacancy dipoles (IV) concentration in KCl: Sr${\mathrm{Cl}}_2$ crystals indicate that the growth of the $Z_2$ centers is accompanied by a decrease of the IV band. The proportionality constant between the number of $Z_2$ centers created and the number of IV dipoles destroyed during the $F\to Z_2$ conversion was slightly more than 3. These results support a model for the $Z_2$ center having spherical symmetry and suggest that the process of formation of the $Z_2$ centers is closely related to the association of IV dipoles into trimers.