Simultaneous measurements of pure and doped (up to 8.1ppm60Co) crystals were carried out at temperatures of between 300 and 730C. The conductivity of the Co-doped crystals was much lower than that of Ca-doped crystals which had a comparable impurity content. The extrinsic conduction in Co-doped crystals was due mainly to the presence of anion vacancies. This, in turn, was attributed to the formation of (CoCl4)2- ions which entered the KCl lattice as an anion impurity. The solubility of the latter was suggested to be less than 10ppm at 730C. The excess Co existed in a precipitated form having the composition, K2CoCl4. The anion migration enthalpy was deduced to be equal to 0.92eV, and the anion impurity-vacancy complex association enthalpy was estimated to be equal to 0.54eV.

D.Damien, K.V.Reddy: Crystal Lattice Defects and Amorphous Materials, 1983, 10[1], 39-46