Unveiling the Origin of Anomalous NIR Emission in Cr-Doped Sr₂P₂O₇: A First-Principles and Experimental Study

Anomalous NIR emission has been reported in Cr³⁺-doped Sr₂P₂O₇, despite the Cr³⁺ ions occupying nonstandard nine-coordinated Sr²⁺ sites. Through first-principles calculations, we demonstrate that these environments induce structural distortions and nonradiative relaxation channels, effectively quenching luminescence. Photoluminescence data and energetic analysis instead attribute the observed emission (∼840 nm) to a coexisting phase, Sr₃(PO₄)₂, where Cr³⁺ substitutes for six-coordinated Sr²⁺, which is successfully verified in experiment. Furthermore, a miniature NIR pc-LED incorporating this phosphor enables biomedical imaging and nondestructive inspection, underscoring its practical relevance.