Abstract
In alkali halides doped with bivalent cations the impurity-vacancy dipoles aggregate by a third-order reaction. Comparison of the activation energy of this reaction with the activation energy of the saturation diffusion coefficient gives the binding energy of a loosely bound dimer which is a precursor to the trimer. In a previously reported case (NaCl:Pb2+), where the guest ion is appreciably larger (20%) than the host, a binding energy of the dimer of 0.13 eV was found. In contrast, in the case reported here (NaCl:Ca2+), where the guest and the host are nearly equal in size, the binding energy of the dimer is not more than 0.02 eV.