Diffusion Constants of Br in NaCl Measured by Rutherford Backscattering Spectroscopy

Rutherford backscattering spectrometry was used to measure the diffusion constants of bromide in NaCl crystals at temperatures from 20 to 600 °C. For crystals grown from melt at 500 °C and above, our data agree very well with the literature data measured with other techniques. At about 450 °C, the activation energy drops strongly leading to a knee in the Arrhenius plot and surprisingly high diffusion constants at room temperature of ∼2×10−16 cm2/s. In case of crystals grown from aqueous solution, cleavage leads to a significant surface enrichment of Br on the newly formed surface compared to bulk composition. Hence, in such porous crystals, Br can move several microns within minutes. Preannealing at 500 °C for 3 h prevents this surface enrichment. Diffusion constants in such preannealed crystals are approximately the same as in melt grown crystals.