Electron-Paramagnetic-Resonance and Optical-Absorption Study of BrCl−and Associated Centers in Doped KCl Crystals. II

Correlated optical and EPR studies have shown that after x or $\gamma$ irradiation and proper thermal treatment, Br${\mathrm{Cl}}^{-}$ molecule ions aligned along the six directions are present in crystals of KCl doped with a small amount of KBr and Pb${\mathrm{Cl}}_2$. When such a crystal is warmed to near 275 K, this Br${\mathrm{Cl}}^{-}$ center is thermally destroyed and another center, a Br${\mathrm{Cl}}^{-}$ with a positive-ion vacancy in a nearest-neighbor position to both nuclei of the molecule ion, is formed. The internuclear axis of the latter center lies in a {100} plane and makes an angle of 12.5° with a direction; the bromine end of the molecule ion lies closest to the vacancy. This Br${\mathrm{Cl}}^{-}$ associated with a positive-ion vacancy has a strong $\sigma$-polarized transition at 368 nm and a weak $\sigma$-polarized transition at 910 nm; optical excitation at 4.2 K can cause preferential orientation of these molecule ions. It is found that this center reorients thermally by making what corresponds, in -oriented Br${\mathrm{Cl}}^{-}$, to 90° jumps at 45 K and what corresponds to 60° jumps at 55 K. The mechanism of formation, the structure, the reorientation, and other properties of this center are discussed.