AGGREGATION OF DIVALENT IMPURITIES IN CODOPED NaCl AND KCl SINGLE CRYSTALS. Jacques Soullard, Instituto de Fisica, UNAM, 01000 Mexico D.F., Mexico

The formation energy of small clusters in NaCl and KCl single crystals codoped with Mn²⁺ and Pb²⁺ impurities has been calculated by means of a static simulation of ionic crystals. The results are used to study quantitatively the structure of the equilibrium state of crystals as a function of temperature.

At high temperature, in both systems, the equilibrium state is characterized by isolated defects: doubly charged substitutional impurities, cation vacancies and dipoles. The manganese dipoles are found in the n.n. and n.n.n. configurations, the ratio of their concentration being dependent on the host matrix. For the lead dipoles the n.n. configuration is dominant in both systems.

In the system NaCl : Mn,Pb dimers and trimers form when the temperature is less than 450K. The nucleation planes of dimers are {110} and {111}, at 300K the trimer concentration is much greater than the dimer concentration and about 50% of trimers have a mixed character.In the system KCl : Mn,Pb, the formation of dimers and trimers becomes significant for temperature less than 500K. The nucleation planes of dimers are {110} and {100}, at 300K the equilibrium state is characterized by a high concentration of dimers, only 10% of dimers are of mixed type.

These results agree qualitatively well with the experimental results. The host matrix where the concentration of closed Mn-Pb pairs is the highest (NaCl) is the most efficient to transfer energy from Pb to Mn; as a consequence, it presents the most intense fluorescence. The differences between the aggregation processes of the two systems can be understood considering the relative solubility of impurities and the binding energy per dipole of clusters.