Diffusion of bismuth ions in potassium chloride crystals

The diffusion of bismuth in single crystals of potassium chloride was measured by a spectrophotometric method. The measurements were carried out in the temperature range 275°-674°C. The Arrhenius plot consists of two straight lines intersecting at about 400°C. The corresponding diffusion coefficients are expressed by D<400°C= 1.7×10^-3 exp(-0.63/kT), D>400°C=5.6×10^-3 exp(-0.97/kT). In the high-temperature region bismuth diffuses in its divalent state. The energy of formation of the diffusing complex Bi²⁺ and vacancy is found to be 0.57 eV. The low-temperature diffusion is that of trivalent bismuth and is attributed to an interstitial mechanism limited by the jump frequency wi. An interstitial mechanism is also proposed for other polyvalent cations with small radii which are able to exist in tetrahedral coordination.