One Shoot Two Birds: Stabilization of Mn in Ba(Si,Ge)O as an Efficient NIR Thermometer and Intense Turquoise Pigment.

Luminescence of transition metal ions in unusual oxidation states is a treasure for developing new phosphors. Pentavalent manganese (Mn), which possesses a 3d electron configuration, has been reported less compared to its Mn and Mn counterparts due to the difficulty in its stabilization. In this study, Ba(SiGe)O:Mn phosphors were synthesized by a solid-state reaction method. The successful Mn stabilization in the (Si,Ge)O tetrahedron leads to a bright turquoise body color with a strong NIR-II sharp-peak emission at room temperature. By replacing Si with Ge, the peak position of the E-A transition remains at 1181 nm, while the corresponding integrated intensity is enhanced by 150% at = 0.6. First-principles density functional theory (DFT) calculations were performed to explore the geometrically optimized structure and electronic structure variation of the Ba(Si,Ge)O:Mn phosphor. Then, they are discussed in association with the observed luminescence properties. It is found that modification of the host composition could modify the accelerated thermal quenching rate of the E emission. Temperature can be read from MnE vs T luminescence intensity ratios. This work represents a step toward exploring the unusual Mn as the emitting center for the next-generation NIR-II phosphors and chromophores for new pigments.