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, Na_2(1–m)K_2mAl_10.8O₁₇:0.2Cr³⁺ and Gd_1–nLa_nMgAl_10.8O₁₉:0.2Cr³⁺, 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.