Behavior of Divalent Impurities in Sodium Chloride Crystals

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Abstract
Electric conductivity measurements in the temperature range from 100° to 500°C were made on sodium chloride crystals doped separately with magnesium, calcium, strontium, and barium as divalent impurities, and on undoped crystals. The results indicate that the mobility of the cation vacancies, the binding energy of the complex and the solubility of impurity in the crystal lattice are dependent on the divalent ion radius, but the jump activation energy is not affected by the impurities. The concentration of unavoidable impurities in the undoped crystal is estimated to be of the order of 7×10−6 molar fraction and they are likely to be calcium. The analysis also shows that to determine the concentration and mobility of vacancies in an undoped crystal by measuring the conductivity as a function of the concentration of divalent ions artificially dissolved in the crystal, it is necessary to use the divalent ions whose physical parameters are close to those of the unavoidable impurity ions.