Is Bi²⁺ Responsible for the Red-Orange Emission of Bismuth-Doped SrB₄O₇?

Red-orange luminescence from bismuth-doped SrB₄O₇ has previously been reported and assigned to 6p-6p transitions of divalent Bi. To provide support for this assignment and for the stability of this unusual valence state of Bi, we report here results from low-temperature luminescence spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and wave function based ab initio calculations. Low-temperature luminescence spectra reveal zero-phonon lines (ZPLs) in excitation and emission spectra, allowing an accurate determination of the energies for the electronic transitions. The influence of the Bi concentration on the emission intensity is shown to be small, and only a small increase of the red-orange emission is observed upon raising the nominal Bi concentration from 0.02% (200 ppm) to 2%. This result indicates that only a very low concentration of Bi²⁺ can be incorporated in SrB₄O₇. This observation is supported by EPR experiments, which do not show a signal that can be assigned to Bi²⁺, and by XANES experiments showing that most Bi is in the trivalent state. An upper limit of the Bi²⁺ concentration is estimated to be 20 ppm. Ab initio calculations on the (BiO₉)^16– cluster embedded in SrB₄O₇ give energies for excited states that are close to the experimentally observed energies. Also, the luminescence lifetime for the red-orange emission (∼12 μs) is consistent with the lifetime for 6p-6p emission calculated for the Bi²⁺ emission (3.5 μs). Equivalent ab initio calculations for Bi²⁺ luminescence are very far from the experimental results, providing independent evidence and additional support for the interpretation of stable Bi²⁺ species being responsible for the red-orange luminescence. The calculations provide a new interpretation of the third excitation band, which is not due to a ²S_1/2 state of the 6s²7s configuration of Bi²⁺, as previously assumed, but is due to a state with important characters of 6s6p²–⁴P (63%) and doublets of the 6s6p², 6s²6d, and 6s²6p configurations; its higher intensity is due to its character of parity-allowed 6s → 6p and 6p → 6d excitations.