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-sp ectra taken in the range 4 K less than or equal to T less than or equa l to 300 K. The observed spectra present numerous temperature-dependen t peaks attributed to the V-F-center, the U-center and to Ca colloids. The defects are unstable at room temperature, the main peaks decreasi ng by an order of magnitude in intensity after a month. Isochronal ann ealing studies revealed a complex evolution of the principal peaks occ urring in two stages between room temperature and 500 degrees C. Paral lel dielectric-constant measurements performed on the same specimens a t room temperature exhibited analogous recovery stages of the real com ponent of the dielectric constant, epsilon'. The results are interpret ed in terms of metal-colloid formation and evolution.