Dielectric and optical properties of lead-activated sodium and potassium chloride crystals

As part of a study of the aggregation of impurity ions in alkali halides, measurements of dielectric loss, of optical absorption and of emission spectra have been made on NaCl: Pb²⁺ and KCl: Pb²⁺ crystals. Changes in the optical absorption and the emission spectra with aggregation of the Pb²⁺-vacancy pairs are more pronounced in NaCl than in KCl. The main conclusions from the NaCl: Pb²⁺ measurements are:

1. changes in the absorption and emission spectra result from the same aggregation process studied previously in many alkali halide crystals by dielectric measurements;

2. the emission spectra, but not the absorption, change when the divalent ion-vacancy pairs (dipoles) form the initial aggregates (trimers); later in the aggregation process both absorption and emission change;

3. the optical measurements show that the aggregation takes place in several definite stages and the results are not inconsistent with the model proposed earlier, viz. the initial aggregates are trimers and these grow into pentamers, and then heptamers etc. by the addition of dipoles, two at a time.

The dielectric absorption measurements give activation energies for the diffusion of Pb²⁺-vacancy pairs through NaCl and KCl lattices of 0·80 ev and 0·85 ev respectively.