Site-specific reduction of Bi3+ to Bi2+ in bismuth-doped over-stoichiometric barium phosphates

The luminescent properties of bismuth-activated materials depend strongly on the exact valence of the prevalent bismuth species. To create bismuth species with valence lower than trivalent, synthesis must usually be performed in a reducing atmosphere which facilitates the reduction reaction. In this work, we propose an alternative approach where interstitial barium sites are intentionally created in a bismuth-doped barium compound with an open crystal structure. We observe that this leads to the auto-reduction of Bi3+ to Bi2+ even when the sample is prepared under oxidizing conditions. We show that the approach can be generalized to various host matrices, i.e., Ba3(PO4)2, Ba2P2O7:Bi and Sr3(PO4)2. It therefore provides a convenient and highly efficient tool for controlling the valence state of bismuth towards exploiting the rich variety of bismuth-based optical materials.