Dipolar Complexes in KCl Resulting from 1.5-MeV-Electron Bombardment

The dielectric relaxation behavior of KCl crystals after exposure to 1.5-MeV electrons has been studied by means of the ionic-thermocurrent (ITC) method. At least four ITC relaxation peaks at 125, 145, 160, and 195 K are observed over the range from 80 to 200 K after irradiation at 260 K. The temperature of irradiation is found to have a strong influence on both the form of the ITC spectrum and the total amount of polarization. The production of dipolar defects, assumed to be responsible for the relaxation, is highest for irradiations in the range 200-260 K and it is rather small for exposures at 150 and 300 K. Once produced, the dipolar defects are relatively stable but they can be nearly completely removed by a 400-K anneal. The effect seems to be intrinsic in nature since the total polarization builds up linearly with exposure after the $F$ center has substantially saturated and since it is essentially the same for high-purity crystals from several ingots. Assuming that unit dipoles are responsible for the ITC spectrum, the initial rate of formation is 0.05 times the initial rate of $F$-center production. Since it is difficult to conceive that such a variety of stable dipoles can be constructed from anion Frenkel defects, it is concluded that defects on the cation sublattice also result from the irradiation. It is shown that the yield of dipoles is at least an order of magnitude higher than can be expected to result from direct displacements induced by electron-ion collisions. The same types of defects may be precursors for the crystallographic voids and dislocation loops observed by electron microscopy in heavily irradiated alkali halides.