It was noted that, following X-irradiation at below 70K, LaF3 exhibited a 3-line electron paramagnetic resonance that was consistent with self-trapped holes (Vk centers) on the F sub-lattice. It was concluded that the Vk centers formed on pairs of F-ions which made angles of 15° with respect to the c-axis. Below 200K, the spectrum of the X-ray excited luminescence was dominated by the 5d-4f transitions of Ce3+ impurities (4ppm). Near to room temperature, the transfer to Ce3+ apparently ceased; revealing a weak band that was suggested to be due to self-trapped exciton luminescence. Above 400K, the 5d-4f transitions of Ce3+ again dominated the spectrum. This implied the continued efficient transfer of energy to the dilute impurity, from uniformly distributed X-ray excitations in the crystal. In CeF3, a 7-line hyperfine electron paramagnetic resonance pattern at g = 2, with an isotropic angular dependence, comprised independent 3-line and 4-line patterns with differing thermal and microwave power saturation properties. The patterns arose from a single electron or hole which interacted with 2 or 3 equivalent spin-½ nuclei, respectively.

EPR and Luminescence Studies of LaF3 and CeF3 under X-Ray and Laser Irradiation. E.D.Thoma, H.Shields, Y.Zhang, B.C.McCollum, R.T.Williams: Journal of Luminescence, 1997, 71[2], 93-104