Dielectric loss, optical absorption spectra and electrical conductivity of lead doped alkali halides

Abstract

Dielectric loss measurements show only one loss peak at a frequency fm at any fixed temperature in the range 80 degrees to 150 degrees C. The values of f0 and E in the equation fm=f0exp(-E/kT) are determined for lead doped KCl, KBr and KI crystals. The electrical conductivity measurements give the values 0.4 eV (KCl), 0.3 eV (KBr) and 0.4 eV (KI) for the energy of association of Pb2+ with a cation vacancy. On colouring the crystals, the dielectric loss peak disappears, the electrical conductivity decreases and new optical bands in the range of 240 to 300 nm are obtained. These results are attributed to the conversion of Pb2+ ions to Pb+ ions on coloration. On heating the KCl:Pb coloured crystals to a temperature in the range 300 degrees -500 degrees C, Pb+ changes to Pb0. On heating the crystals to higher temperatures, Pb colloidal particles are formed. Pb0 centres give rise to an absorption band at 214 nm and Pb colloidal particles, one at 240 nm.