Lead Centers in Cesium Halides

Lead as an impurity in cesium halides gives the usual A, B, and C bands with some differences, such as the appearance of a doublet structure in the A band and the observation of an aggregate lead complex band in the visible region, when compared with the results for the other lead-doped alkali halides. In this paper the peak positions, half-widths, and dipole-strength ratios of these lead centers in CsCl, CsBr, and CsI are reported. In irradiated cesium halides containing divalent lead, a new band is observed in the uv region at about 20 nm to the shorter-wavelength side of the A band. This band is interpreted to be due to Pb⁺ from various optical absorption studies and is found to be stable up to 110^∘{}C. In crystals irradiated for a longer time, this Pb⁺ band goes down along with the Pb⁺⁺ bands and a new band is observed which is attributed to Pb⁰. It has been found from conductivity studies that the binding energy for an impurity-vacancy pair in cesium halides is about 0.48 eV and the presence of divalent lead in these crystals decreases the conductivity in the extrinsic region. From the dielectric-loss measurements, the migration energy for a cation vacancy bound to the impurity is found to be {~}0.62 eV while the preexponential factor is of the order of 3×{}10¹⁰ sec^-1.