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.