F and Vk centres in LaF3, CeF3 crystals
Optical absorption properties of additively colored in calcium vapor and X-irradiated crystals with tysonite structure, LaF3 and CeF3, were studied in 1.5-5.5 eV energy range. Photobleaching with 1.9 eV light and dichroism of the absorption bands were investigated.The absorption band at 3.75 eV with half width of 1.1 eV is observed in X-irradiated LaF3, similar to the Vk-centre-bands observed in other fluorides. No such band is observed in X-irradiated CeF3. In additively colored CeF3 absorption bands at 1.7 eV, 2.15 eV are observed. These bands can be attributed to F centres. Additive coloration upon heating in calcium vapor did not produce any new absorption band in LaF3 crystals while formation of metallic inclusions, most likely La precipitates were observed. For X-irradiated LaF3 and CeF3, at temperature below 110K the photo-bleachable, dichroic bands are observed: near 2 eV -band perpendicular to c-axis, 2.6-2.7 eV -band along c-axis. F centres occupying the vacancies of fluoride F(1) layers in X-irradiated crystals and F-centres occupying all fluoride vacancies in additively colored crystals are proposed to be observed