Unlocking color-tunable emission of Eu2+-activated phosphors through doping-free exploration of hidden sites.

While numerous approaches have been devised to optimize the luminescence properties of Eu²⁺-activated phosphors, the quest for a simple design strategy to achieve color-tunable Eu²⁺ emission remains a significant challenge. Here, we present a straightforward, doping-free method to tailor the site occupation of Eu²⁺ and modify its luminescence properties. In the pristine sample, the NaBaB₉O₁₅ (NBB) host featuring two cation sites (Na⁺ and Ba²⁺), Eu²⁺ exclusively occupies the Ba²⁺ site, resulting in blue emission when the NBB:Eu²⁺ phosphor is synthesized in a H₂ reducing atmosphere. When we use CO instead of a H₂ reducing atmosphere, impressively, the formation energy required for heterovalent substitution of Na⁺ by Eu²⁺ is lowered, leading to the emergence of a distinct green emission band. Significantly, the resultant phosphor NBB:Eu²⁺@CO exhibits not only a high internal quantum yield of 85.1% and zero-thermal-quenching behavior within the temperature range of 25–200 °C but also wavelength-tunable emission. Benefitting from the multi-color luminescence of the NBB:Eu²⁺@CO phosphor, its optical applications will no longer be confined to general lighting but extend to anti-counterfeiting and encryption. This study provides a novel design methodology for optimizing the optical properties of Eu²⁺-activated phosphors. [ABSTRACT FROM AUTHOR]