Diffusion of Lead Ions in Purified NaCl Crystals

The diffusion of Pb2+ in purified NaCl has been studied over the temperature range from 348° to 553°C by a radiotracer technique. Measurement of the concentration-dependent diffusion coefficient provides an estimate of the free energy of association for the Pb2+-cation vacancy complex. It was found to be 0.35 eV at 553°C and increase to 0.48 eV at 348°C. As expected the free energy of association was lower in these purified crystals than that previously measured in commercial crystals. The saturation diffusion coefficient, Dδ, is given by Ds = 0.0175 exp ( − 22 800 cal / RT) over this temperature range. The results of these experiments agree with the previous work on the NaCl-PbCl2 system and support the idea that lead ion impurities are more stably bound to a substitutional cation site in KCl than NaCl.