Polycrystalline samples were irradiated with 1MeV electrons at temperatures ranging from 21 to 275K, and their electron paramagnetic resonance spectra were measured. It was found that the created defects exhibited a marked dependence upon the irradiation temperature. At temperatures below 200K, mainly F+ centers (O vacancies with a trapped electron) were formed. Heat treatment led to their recovery at temperatures near to 400C. This was accompanied by the appearance of a new signal near to g = 2.003. This was suggested to be due to F+ agglomeration into small clusters. The latter disappeared at annealing temperatures of between 600 and 700C. At an irradiation temperature of 200K, the F+ spectra were superposed by the g = 2.003 line, and by a new narrow signal at g = 2.0023 which was attributed to small metallic colloids. At an irradiation temperature of 275K, only a narrow temperature-independent line (about 0.01mT) was observed at g = 2.00235. This was characteristic of metallic Li colloids. The microwave dielectric constant of the same specimens increased appreciably after room-temperature irradiation, thus emphasizing the presence of metallic particles. It remained unchanged at lower irradiation temperatures. Both the colloid line and the radiation-induced dielectric constant change vanished simultaneously upon annealing at temperatures above 250C.

P.Vajda, F.Beuneu: Physical Review B, 1996, 53[9], 5335-40