Abnormal Lattice Shrinkage, Site Occupation, and Luminescent Properties of Cr3+-Activated β-Al2O3 Structure Phosphors

β-Al₂O₃ phosphors show good optical properties for highly symmetric lattices and dense frameworks. Here, an abnormal lattice shrinkage phenomenon is found in BaMg_1-xZn_xAl_9.8O₁₇: Cr³⁺ (BM_1-xZ_xA: Cr³⁺) phosphors, which can be attributed to the variation in the covalency of the chemical bond. Accordingly, the denser crystal structure caused by the abnormal shrinkage improves the luminescent intensity (67%↑) and the thermal stability (69%→76%@150 °C). Internal/external quantum efficiency (IQE/EQE) of BZA: 0.2Cr³⁺ reaches 95% and 52.7%, respectively. In addition, the preferential occupation of the Cr³⁺ ion is discussed through the crystal field strength calculation, low-temperature spectra, and fluorescent lifetime. The unique spectrum of the phosphor derives from the occupation of Cr³⁺ on the octahedral sites (Al4 and Al1) and the formation of Cr³⁺-Cr³⁺ coupling pairs. Finally, the high matching rate between the absorption curve of plant pigment P_fr and the EL spectrum of the as-prepared pc-LED expresses that it can be applied in plant lighting.