Simultaneous diffusion of calcium and strontium in KCl single crystals

Abstract

Concentration dependent diffusion coefficients for 45Ca 2+ and 85Sr 2+ in purified KCl were measured using a sectioning method. KCl was purified by an ion exchange — Cl 2−HCl process and the crystals grown under 1 6 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 451% to 669%C. At temperatures above 580%C (the lowest melting eutectic in this system) diffusion was from a vapor source: below 580%C surface depositied 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 KCl the saturation diffusion coefficients D S (ca) and D s (Sr) are given by D s(Ca) = 9.93 × 10 −5 exp(−0.592 eV kT ) cm 2 sec (1) and D s(Sr) = 1.20 × 10 −3 exp(−0.871 eV kT ) cm 2 sec (2) for calcium and strontium, respectively. The Gibbs free energy of association of the impurity vacancy complex in KCl for calcium can be represented by Δg(Ca) = ∂-0.507 eV + (2.25 × 10 −4 eV %K )T (3) and that for strontium by Δg(Sr) = −0.575 eV + (2.90 × 10 −4 eV %K )T . (4)