Optimizing the microstructure and luminescence of GdYAG:Ce: A study of the pH and annealing temperature cross-effect

This study investigates the synthesis and characterization of cerium-doped mixed garnet Gd₂.₆₇Y₀.₃Al₅O₁₂ (called GdYAG: Ce) prepared via the sol–gel method. The effects of pH (pH = 1, 3, 5, 7, and 9) and annealing temperature (800–1500 °C) on the stabilization of the garnet phase were studied and discussed. The results reveal that pH strongly influences phase stabilization, with the garnet structure predominantly forming under acidic or basic conditions, whereas intermediate pH values lead to the formation of the perovskite phase. Additionally, the effect of annealing temperature on samples prepared at pH = 1 indicated that the amorphous state observed at 800 °C gradually transforms into an hexagonal GdAlO₃ phase at 900 °C, followed by mixed cubic garnet and perovskite phases between 1000 °C and 1300 °C, and finally a single well-defined cubic garnet phase at 1500 °C. FESEM analysis reveals that increasing the annealing temperature leads to particles with more well-defined morphologies and a narrower size distribution. Photoluminescence results indicate the absence of Ce³ ⁺ emission at 800 °C, attributed to the amorphous nature of the GYAG lattice. As the annealing temperature increases, the characteristic Ce³ ⁺ transitions are observed, and the luminescence intensity gradually rises up to 1300 °C before decreasing at 1500 °C, likely due to excessive crystallite growth and enhanced non-radiative quenching.