Cr³⁺-Activated Spinel Phosphors with Inhibited Cr(VI) Formation for Multiapplication via a Lattice Occupation...

Cr³⁺-activated phosphors are prevalent in red to near-infrared (NIR) luminescent applications but face the contamination problem of Cr⁶⁺ coexistence. This work presents a competitive site occupation strategy in spinel-type MAl₂O₄:Cr³⁺ (M = Zn, Mg) by introducing tetrahedral structural units, including [BO₄] and [SiO₄], to occupy the tetrahedral sites, thereby preventing Cr³⁺ from entering and oxidizing to Cr⁶⁺. This preferred tetrahedral occupancy exhibits excellent universality in the spinel system, eliminating Cr⁶⁺ formation at its source. Moreover, the photoluminescence properties of the phosphors are greatly improved. Among them, ZnAl_1.78B_0.2O₄:Cr³⁺ (ZABO:Cr) has over five times the emission intensity of pristine ZnAl₂O₄:Cr³⁺ (ZAO:Cr). The internal quantum efficiency (IQE) and external quantum efficiency (EQE) increase from 10.03 and 5.62% for ZAO:Cr to 81.31 and 35.87% for ZABO:Cr, respectively. Furthermore, [BO₄] substitution increases the electron-trapping defect concentration, which endows ZABO:Cr with thermal quenching resistance and X-ray-responsive photochromism. The multistimuli response and multipeak far-red to NIR emission of this material provide versatile application prospects, including plant cultivation, night vision, biological imaging, optical thermometry, and anticounterfeiting. This study proposes a general method to stabilize the Cr³⁺ valence state in Cr³⁺-doped materials while enhancing their functional properties and opening new application avenues.