Structural Design of Cr³⁺-Activated Hexaaluminate Phosphors with High Quantum Efficiency and Cr³⁺–Cr³⁺ Exchange Coupling Pairs

Introducing large-radius cations usually causes structural relaxation, leading to a spectral redshift and low optical performance in Cr3+-activated phosphors. Here, MNAl10O17:Cr3+ phosphors are synthesized, and the substitution of large-radius cations induces the abnormal lattice shrinkage of the N site and atomic site splitting of the M site due to the unique hexaaluminate structure, further distinctly improving luminescent properties. To investigate the university of atomic site splitting, a series of phosphors, Na2(1–m)K2mAl10.8O17:0.2Cr3+ and Gd1–nLanMgAl10.8O19:0.2Cr3+, are synthesized, and the variation of their luminescent properties conforms to the expected rule. Finally, Ga3+ ions are introduced to improve the luminescence efficiency. Internal/external quantum efficiencies of the optimal sample are 98.1 and 65.8%, respectively. Meanwhile, an anomalous spectral blueshift indicates the existence of Cr3+–Cr3+ exchange coupling pairs, and a comparative analysis of similar cases is conducted to provide some insights into the luminescence of coupling pairs.