Color Centers in TlCl

Although TlCl is thought to contain an equilibrium number of anion vacancies which is comparable with the alkali halides, no $F$ centers can be produced in pure TlCl, either by x-ray irradiation at 100 or 300°K, or by addition of excess Tl metal. Addition of chalcogen impurities, however, produces an absorption peak in crystals quenched from high temperature. This peak lies at 409 mμ in TlCl: ${\mathrm{Tl}}_2$S and at 420 mμ in TlCl: ${\mathrm{Tl}}_2$Se, with oscillator strengths of 0.22 and 0.27, respectively; the peak narrows and the position shifts when the absorption is measured at lower temperature. The direction of the temperature shift is unusual, in that it moves toward longer wavelength at low temperature. As the quenched crystal approaches equilibrium at room temperature, the peak transforms into another band whose peak is hidden by the fundamental absorption. This band is identical for the sulfur- and selenium-doped crystals, and it is independent of the temperature of measurement. At intermediate temperatures, the equilibrium absorption is a mixture of the two bands, which have an isosbestic point at 455 mμ. At room temperature the transformation occurs in the dark, although its rate is accelerated by illumination with white light. In both cases the rate is faster for the sulfur-doped crystals than for the selenium-doped ones. Similar bands are observed in TlBr: ${\mathrm{Tl}}_2$S and TlBr: ${\mathrm{Tl}}_2$Se, although in this case all the peaks are hidden by the fundamental absorption. The room-temperature absorption, which is independent of the impurity and of temperature, is interpreted as due to colloidal Tl metal. The high-temperature peak, which is capable of reversible transformation into the colloid band, is interpreted as due to $F$ centers associated with ${\mathrm{S}}^{-}$ or ${\mathrm{Se}}^{-}$ impurity ions. The transformation to the colloid absorption results from coagulation of the $F$ centers. With this explanation, some comparisons are drawn with $F$ centers in the cesium halides on one hand, and with the impurity absorption bands in chalcogen-doped silver halides on the other hand.