Self-Diffusion of the Chloride Ion in Sodium Chloride

The diffusion coefficient of Cl³⁶ in NaCl has been measured in the temperature range from 520^∘{} to 740^∘{}C. Diffusion was measured in pure Harshaw crystals and in crystals containing from 0.01 to 0.1 mole percent calcium. The diffusion coefficient in pure crystals is represented by the equation D=56exp(-2.12 ev/kT) cm²/sec. The diffusion coefficient in crystals containing calcium was smaller than that measured in pure crystals by a factor of from 5 to 10, and had an activation energy of 2.5 ev. Over the range of impurity concentration employed the diffusion coefficient was insensitive to differences in calcium concentration. The results are discussed in terms of motion of free negative ion vacancies and of vacancy pairs. Possible complicating effects of dislocation lines on the diffusion coefficient are also considered.