Persistent Luminescence in Highly Nonstoichiometric GAGG Garnets Gd _3+x [Al ₂ Ga ₃ ] _1‐x/5 O ₁₂ (0 ≤ x ≤ 0.6) Doped with Ce ³⁺ /Cr ³⁺

Abstract The gadolinium garnet Gd ₃ Al ₂ Ga ₃ O ₁₂ co‐doped with Ce ³⁺ and Cr ³⁺ (GAGG‐Ce,Cr) has been widely studied due to its unusual bright yellow long‐lasting persistent luminescence properties. Here, rapid containerless melt‐quenching is used as part of a two‐step glass‐crystallisation synthesis process to obtain a new highly nonstoichiometric form of this garnet, of composition Gd _3+x [Al ₂ Ga ₃ ] _1‐x/5 O ₁₂ with 0 ≤ x ≤ 0.6 (ns‐GAGG). For compositions x > 0, powder X‐ray diffraction analysis confirms that excess Gd ³⁺ is accommodated at the Al ³⁺ /Ga ³⁺ sublattice in octahedral coordination, by substituting up to 30% of these sites. This mode of substitution complexifies the local structure of the garnet host, which is shown to influence certain luminescence properties in the analogous highly nonstoichiometric Y _3+x Al _5‐x O ₁₂ (0 < x < 0.4) and Gd _3+x Al _5‐x O ₁₂ (0 < x < 0.6) systems. Co‐doping ns‐GAGG with Ce ³⁺ and Cr ³⁺ produces green‐yellow persistent luminescence when x = 0, which undergoes a redshift to yellow‐orange as the Gd ³⁺ content increases to x = 0.4. However, this radical modification of the host composition does not strongly affect the afterglow kinetics. These results demonstrate an effective and high‐precision way of decoupling color and kinetics in persistent luminescent garnets, which is usually hard to achieve using the standard stoichiometric material engineering approach.