X-ray Induced Valence Change and Vacuum Referred Binding Energies of Bi3+ and Bi2+ in Li2BaP2O7

Bismuth-doped Li2BaP2O7 was prepared in air, showing no Bi3+ or Bi2+ related photoluminescence even at 10 K. Absorption measurements showed that only Bi3+ was present in the as-prepared samples of which the emission is completely quenched. During X-ray excitation the characteristic deep-red radioluminescence of Bi2+ was observed. After X-ray irradiation, this red luminescence of Bi2+ could be excited optically, indicating that upon X-ray irradiation the Bi3+ is reduced to Bi2+. Based on the spectroscopic results, the Bi3+ and Bi2+ energy levels were estimated in a vacuum referred binding energy (VRBE) scheme and were used to explain the observed luminescence behavior. The VRBE scheme provided an interpretation for the commonly observed Bi3+ pair emission in bismuth-doped compounds. In the case of Li2BaP2O7:Bi it was used to explain the self-quenching behavior of Bi3+. These findings show that is possible to initially dope compounds with Bi3+ ions while only radioluminescence is observed from Bi2+ when the sample is exposed to high-energy excitation. This phenomenon can be used to fabricate new types of luminescent materials.