Perturbed (F2-) centres in KZnF3

Optical and ESR experiments on gamma-irradiated KZnF 3 crystals show that three types of perturbed (F 2 - ) centres are produced at 77K. These defects basically consist of a trapped hole equally shared by two neighbouring fluorine ions, and are similar to the (X 2 - ) centres in the alkali halides. The thermally induced decay of the three centres occurs at 150, 225 and 310K. The corresponding optical absorption for each of the centres is at 3.27, 3.33 and near 3.45 eV. Anisotropic absorption induced in the crystals by preferential alignment is observed in the optical spectra of the centres decaying at 225 and 310K. These two centres are believed to be stabilized by impurity ions. The centre decaying at 150K exhibits rapid motional behaviour in the form of a pronounced ESR line broadening in the temperature range above 92K. The failure to observe preferential alignment at 77K in the optical absorption spectrum of this defect also suggests rapid motional behaviour. A plausible model compatible with these results is that of the fluorine molecular ion located adjacent to a cation vacancy.