EPR-study of electron-radiation induced Ca colloids in CaF2 crystals

Oriented single-crystal specimens of CaF2 have been irradiated at room temperature with electrons of 0.5 and 1 MeV energies and their EPR-spectra taken in the range 4 K less than or equal to T less than or equal to 300 K. The observed spectra present numerous temperature-dependent peaks attributed to the V-F-center, the U-center and to Ca colloids. The defects are unstable at room temperature, the main peaks decreasing by an order of magnitude in intensity after a month. Isochronal annealing studies revealed a complex evolution of the principal peaks occurring in two stages between room temperature and 500 degrees C. Parallel dielectric-constant measurements performed on the same specimens at room temperature exhibited analogous recovery stages of the real component of the dielectric constant, epsilon'. The results are interpreted in terms of metal-colloid formation and evolution.