Enhancing Luminescence and Controlling the Mn Valence State of Gd3Ga5–x–δAlx–y+δO12:yMn Phosphors by the Design of the Garnet Structure

Gd3Ga5–x–δAlx–y+δO12:yMn solid solutions with improving luminescence properties were prepared via cation substitution and a controllable Mn valence state. The abnormal autoreduction from Mn4+ to Mn2+ ions was observed during the formation of Gd3Ga5–x–δAlx–y+δO12:yMn. The doped manganese ions occupy octahedral Ga3+(1) and Al3+(1) sites to form the Mn2+ luminescent center with red emission at 630 nm and Mn4+ luminescent centers with deep red light emission at 698 nm, respectively, matching well with the red light absorption of phytochrome (PR) and the far-red light absorption of phytochrome (PFR). With the design of the concentration of Al3+ and doped manganese ions, the photoluminescence (PL) of Mn4+/Mn2+ (corresponding to PFR/PR) can be tuned, which is very useful for controlling the plant growth. Moreover, the PL intensity of Gd3Ga5–x–δAlx–y+δO12:yMn can be increased by 6.8 times by substituting Al3+ for Ga3+. The thermal stability is also enhanced significantly. Finally, a series of warm white-light-emitting diodes (WLEDs) with good performance were fabricated using the as-prepared Gd3Ga5–x–δAlx–0.012+δO12:0.012Mn phosphor. The results show that the designed Gd3Ga5–x–δAlx–y+δO12:yMn phosphors have potential practical values in plant-growth light-emitting diodes (LEDs) and high-performance WLEDs. Moreover, our strategy not only provides a unique inspiration for tuning the valence states of Mn but also designs new advanced luminescent materials.