One Shoot Two Birds: Stabilization of Mn5+ in Ba2(Si,Ge)O4 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 (Mn⁵⁺), which possesses a 3d² electron configuration, has been reported less compared to its Mn²⁺ and Mn⁴⁺ counterparts due to the difficulty in its stabilization. In this study, Ba₂(Si_1–xGe_x)O₄:Mn⁵⁺ phosphors were synthesized by a solid-state reaction method. The successful Mn⁵⁺ stabilization in the (Si,Ge)O₄ tetrahedron leads to a bright turquoise body color with a strong NIR-II sharp-peak emission at room temperature. By replacing Si⁴⁺ with Ge⁴⁺, the peak position of the ¹E–³A₂ transition remains at 1181 nm, while the corresponding integrated intensity is enhanced by 150% at x = 0.6. First-principles density functional theory (DFT) calculations were performed to explore the geometrically optimized structure and electronic structure variation of the Ba₂(Si,Ge)O₄:Mn⁵⁺ 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 ¹E emission. Temperature can be read from Mn⁵⁺¹E vs ³T₂ luminescence intensity ratios. This work represents a step toward exploring the unusual Mn⁵⁺ as the emitting center for the next-generation NIR-II phosphors and chromophores for new pigments.