Disorder-Induced Broadband Near-Infrared Persistent and Photostimulated Luminescence in Mg₂SnO₄:Cr³

Materials with near-infrared (NIR) persistent luminescence (PersL) and NIR-to-NIR photostimulated luminescence (PSL) properties are attractive platforms for photonic energy harvesting and release. In this work, we develop Mg₂SnO₄:Cr as a broadband NIR PersL and NIR-to-NIR PSL material (luminescence maxima at ∼800 nm) and reveal the origin of the PersL and PSL properties. The material has an inverse spinel structure with the Mg²⁺ and Sn⁴⁺ disorder at the Wyckoff 16d site based on the Rietveld refinement. Cr K-edge X-ray absorption near-edge structure (XANES) spectra uncover that the doped Cr ions have a +3 valence state and occupy the disordered (Mg,Sn) site with octahedral coordination. The disorder results in multiple Cr³⁺ centers, and the broadband luminescence originates from the ⁴T₂(⁴F) → ⁴A₂ transition of Cr³⁺ at sites with intermediate crystal field strength. The distribution of trap depths is continuous according to the analysis of thermoluminescence (TL) spectra using the initial rising method, which relates to the random distribution of Mg²⁺ and Sn⁴⁺ at the second coordination sphere of the Cr³⁺ centers rather than the oxygen-related defects. Stimulating the material with a NIR laser, the NIR PersL gets significantly enhanced due to a PSL process. The broadband PersL and PSL are detectable beyond 100 h and have good tissue penetrability and therefore the developed Mg₂SnO₄:Cr³⁺ has potential in applications of optical information storage/reading and autofluorescence-free bioimaging. Finally, three crystal and electronic structure factors are proposed for screening new Cr³⁺-activated PersL and PSL materials.