Diffusion of Divalent Impurities from the Volume to the Surface in NaCl Crystals

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Abstract
In NaCl: Ca2+, NaCl: Cd2+, and NaCl: Cu2+ crystals heated in air at temperatures of 400 to 450 °C during 6 to 12 h the impurities tend to migrate to the crystal surface. There the impurities react with air and a new phase is produced on the NaCl surface. Both, transmission and scanning electron microscopy is used to study the structure and characteristics of the new phase. It is found that for NaCl: Ca2+ and NaCl: Cd2+ the new phase corresponds to CaO and CdO, respectively. The phenomenon is discussed using the theory of a double electrical layer that arises in the vicinity of the surface in ionic crystals. However this theory can not explain it completely. Diffusion to the surface must be taken into account in every work that involves a thermal treatment of a ionic crystal containing divalent impurities.