SIMULTANEOUS DIFFUSION OF CALCIUM AND STRONTIUM IN NaCl SINGLE CRYSTALS

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Abstract
Concentration dependent diffusion coefficients for 45Ca2+ and 85Sr2+ in purified NaCl were measured using a sectioning method. NaCl was purified by an ion exchange - Cl2-HCl process and the crystals grown under 116 atmosphere of HCl. The tracers were purified on small disposable ion exchange columns to remove precessor and daughter impurities prior to use in a diffusion anneal. Isothermal diffusion anneals were made in the temperature range from 448 °C to 683 °C. At temperatures above 500 °C (the lowest melting eutectic in this system) diffusion was from a vapor source ; below 500 °C surface deposited sources were used. The saturation diffusion coefficients, enthalpies and entropies of impurity-vacancy associations were calculated using the common ion model for simultaneous diffusion of divalent ions in alkali halides. In NaCl the saturation diffusion coefficients Ds(Ca) and Ds(Sr) are given by Ds(CA) = 1.14 × 10-3 exp(- 0.851 eV/kT) cm2/s and Ds(Sr) = 2.30 × 10-3 exp(-0.925 eV/kT) cm2/s for calcium.and strontium, respectively. The Gibbs free energy of association of the impurity vacancy complex in NaCl for calcium can be represented by Ɗg(Ca) = -0.752 eV + (2.69 × 10-4 eV/K) T and that for strontium by Ɗg(Sr) = -0.671 eV + (3.35 × 10-4 eV/K/T

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