Vacancy-mediated stabilization of Cr3+ and near-infrared luminescence enhancement in Mg2GeO4 via Zn2+ incorporation

Abstract

Near-infrared (NIR) phosphors based on Cr3+ ions have attracted considerable attention owing to their broadband emission and potential applications in spectroscopy, night vision, and optical sensing. However, the practical performance of Cr-activated oxide phosphors is often limited by the valence instability of chromium ions, where partial oxidation of Cr3+ to Cr4+ during high-temperature synthesis leads to competitive emission and reduced NIR-I efficiency. In this work, a vacancy-mediated defect-regulation strategy is proposed to stabilize the Cr3+ valence state and enhance NIR-I emission in Mg2GeO4:Cr phosphors. Zn2+ incorporation effectively suppresses the formation of Cr4+ and promotes the preferential occupation of Cr3+ at octahedral Mg sites, resulting in significantly enhanced broadband emission centered at ~900 nm. The optimized composition, Mg1.59Zn0.4Cr0.01GeO4, exhibits a 5.5-fold increase in integrated emission intensity with an internal quantum efficiency (IQE) of 13.4%. Mechanistic analysis suggests that Zn2+ incorporation lowers the formation energy of charge-compensating magnesium vacancies, which shift the defect equilibrium and thermodynamically favor the stabilization of Cr3+ centers. Furthermore, Li+ codoping occupies these vacancies and passivates nonradiative centers, leading to a further improvement of IQE to 26.8%. This work highlights the important role of intrinsic defect chemistry in regulating activator valence states and provides a useful strategy for designing high-efficiency Cr3+-activated NIR phosphors.