Localized excitons in Fluoroperovskite LiBaF₃ crystals

Two radiating processes in LiBaF₃ crystals, fast valence-core transitions (5.4-6.5 eV) and slow, so called self-trapped exciton luminescence (about 4.3 eV), are important for practical application. Here we present a study of 4.3 eV luminescence under X-ray excitation and photoexcitation as well as under photostimulation after X-irradiation of undoped and Ag-doped LiBaF ₃ crystals at various temperatures. It is shown that 4.3 eV luminescence appears under X-ray excitation at least from 85 K to 400 K in both undoped and doped crystals. In all samples studied the excitation spectra of 4. 3 eV luminescence contain both the main exciton like band at the edge of fundamental absorption at about 10 eV and weaker band in 7.8-8.6 eV region. Luminescence spectrum in the 3.8-4.8 eV region under 7.8-8.6 eV excitation differs slightly from that under 10 eV excitation. Several luminescence bands in 3.8-4.8 eV region arises in the temperature range 85-230 K under photostimulation in absorption band of F-type center at 2.9 eV created previously under X-irradiation. We propose the luminescence of LiBaF ₃ crystals in the 3.8-4.8 eV region may be caused by localized excitons formed not only under excitation near the fundamental absorption but also in result of electron recombination with localized holes thermally destroyed above 230 K.