EPR and luminescence studies of LaF3 and CeF3 under X-ray and laser irradiation

After X-ray irradiation below 70 K, LaF3 exhibits a 3-line EPR resonance consistent with self-trapped holes (Vk centers) on the fluorine sublattice. Specifically, we conclude that the V-k centers are formed on pairs of fluorine ions making 15 degrees angles to the c-axis. At temperatures below 200 K, the spectrum of X-ray excited luminescence is dominated by 5d-4f transitions of Ce3+ impurity present at 4 ppm. Near room temperature, the transfer to the Ce3+ ceases within detectability, revealing a weak band suggested in other work to be self-trapped exciton luminescence. Above 400 K, the 5d-4f transitions of Ce3+ again dominate the spectrum, implying resumption of efficient energy transfer from the uniformly distributed X-ray excitations in the LaF3 crystal to the dilute impurity. In CeF3, a 7-line hyperfine EPR pattern at g = 2, with isotropic angular dependence, is shown to be composed of independent 3-line and 4-line patterns possessing different thermal and microwave power saturation properties. Such patterns arise from a single electron (or hole) interacting with 2 and 3 equivalent spin-1/2 nuclei, respectively.