Lead Centers in Cesium Halides

Abstract

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 ${\mathrm{Pb}}^{+}$ from various optical absorption studies and is found to be stable up to 110\ifmmode^\circ\else\textdegree\fi{}C. In crystals irradiated for a longer time, this ${\mathrm{Pb}}^{+}$ band goes down along with the ${\mathrm{Pb}}^{++}$ bands and a new band is observed which is attributed to ${\mathrm{Pb}}^{0}$. 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 \ensuremath{\sim}0.62 eV while the preexponential factor is of the order of 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{sec}}^{\ensuremath{-}1}$.