Multiple Coordination of Chromium Ion Luminescence: A Strategy for Designing Ultra-broadband NIR Long Persistent Luminescent Materials

Near-infrared (NIR) long persistent luminescent (LPL) materials have attracted the interest of many researchers as they have potential applications in many aspects. However, majority of studies on Cr³⁺ ion-doped LPL materials have focused on Cr³⁺ in an octahedral site, and the luminescence is limited to the short-wavelength NIR-I region (700-900 nm), which is detrimental to fully explore Cr³⁺ ion-doped LPL materials with potential applications. In this work, a novel ultra-broadband NIR LPL material, Na₂CaGe₆O₁₄ (NCGO):x%Cr³⁺, was successfully designed and synthesized, covering the luminescence range of 600-1200 nm and having the best afterglow duration of more than 10 h. Combining the luminescence lifetime with the low-temperature spectrum, it was concluded that the luminescence of NCGO:Cr³⁺ consists of the co-emission of Cr³⁺ in octahedra and tetrahedra. And it was confirmed by electron paramagnetic resonance (EPR) spectrum and X-ray absorption near-edge spectrum (XANES). The application prospects of NCGO:x%Cr³⁺ in many aspects were investigated in detail. This work could not only give a reference for researchers to study Cr³⁺ luminescence in multiple coordination but also provide a new strategy for obtaining new ultra-broadband NIR LPL materials.