Broadband NIR photoluminescence from Bi-doped Ba$_2$P$_2$O$_7$ crystals: Insights into the nature of NIR-emitting Bismuth centers

Abstract

We report on a novel type of Bi-doped crystal that exhibits ultrabroadband photoluminescence in the near infrared (NIR). Emission centers can be generated and degenerated reversibly by annealing the material in CO atmosphere and air, respectively, indicating that emission is related to the presence of Bi-species in low valence states. Correlating static and dynamic excitation and emission data with the size and charge of available lattice sites suggests that two types of Bi0-species, each located on one of the two available Ba2+ lattice sites, are responsible for NIR photoemission. This is further confirmed by the absence of NIR emission in polycrystalline Ca2P2O7:Bi and Sr2P2O7:Bi. Excitation is assigned to transitions between the doubly degenerated ground state 4S3/2 and the degenerated excited levels 2D3/2, 2D5/2 and 2P1/2, respectively. NIR emission is attributed to 2D3/2→4S3/2. The NIR emission center can coexist with Bi2+ species. Then, also Bi2+ is accommodated on one of the two Ba2+-sites. Energy transfer between Bi2+ ions occurs within a critical distance of 25.9 Å.