Near-infrared emission of Cr3+ in five-coordinated field

Cr3+-activated NIR phosphors have received increasing attention due to their versatile applications. Cr3+ generally shows tunable NIR emission by occupying a six-coordinated field (Oh). Herein, we discover broadband NIR emission of Cr3+ at the five-coordinated field in the phosphors BaXP2O7:Cr3+ (X = Mg/Zn) for the first time. The five-coordinated [MgO5] (D3h) or [ZnO5] (C4v) site provides a quasi-octahedral field. The symmetry of D3h and C4v fields is lower than that of Oh, thus leading to further splitting of the energy levels and generating the broadband NIR emission from 700 to 1100 nm. This work breaks through the traditional limitation of NIR luminescence of Cr3+ in the six-coordinated field and opens a new window to explore novel NIR phosphors of Cr3+ in five-fold coordination.