Diffusion of Tl + in Doped and Pretreated KCl

The diffusion coefficients of Tl + ions are measured in “pure” KCl crystals (obtained from the melt or crystallized from aqueous solutions), and in KCl crystals doped with SrCl 2 or BaCl 2 , over the temperature range 160 to 210 °C. Doping and various other treatments decrease the diffusion constants compared with those in the melted sample. This is accompanied by a large increase in the apparent jump energy EA. The assumption of metastable supersaturated solutions of the divalent impurity ions present in “pure” KCl leads to a consistent and quantitative interpretation of these results. The jump energy of the Tl + ions, δh Tl , calculated from results for the above KCl samples is found to be (0.84 ± 0.02) eV, whilst a specially purified sample, containing a total divalent impurity concentration cp = 2.17 × 10 −6 , gives the direct value: δh Tl = 0.836 eV. The mean heat of dissolution χ i of the prevalent divalent impurities is (1.44 ± 0.04) eV, and the corresponding entropy factor η i is 0.953 × 10 −3 eV/deg. The supersaturation concentration in samples from the melt is found to be about 5 × 10 −6 mole fraction.