Synthesis, Structural and Luminescence Properties of Mn4+-Activated Medium-Entropy Perovskites (La1/3Gd1/3Y1/3)1−xMxAlO3−δ (M = Ca, Sr)

Medium-entropy aluminate perovskites of nominal composition (La1/3Gd1/3Y1/3)1−xMxAlO3−δ:0.1%Mn4+ (x = 0, 0.05, 0.1; M = Ca, Sr), were prepared by sol-gel and solid-state routes to examine the effects of synthesis, annealing temperature and A-site substitution on their luminescence. In the case of alkaline-earth-free (La1/3Gd1/3Y1/3)AlO3 system, single-phase materials are formed after heat treatment of sol-gel derived products at the temperature of 1000–1100 °C. Calcium-substituted samples prepared via the sol-gel method are single-phase over a wide annealing temperature range of 1000–1500 °C, whereas strontium-containing samples contain a minor amount of the secondary-phase RESrAl3O7. Solid-state products are multiphase materials, containing perovskite and garnet phase, with extra RECaAl3O7 phase in Ca-substituted samples. The unchanged emission position and spectral profile indicate that Mn4+ ions occupy the same octahedral coordination environment of the perovskite structure in all samples. Moreover, no additional spectral features associated with a garnet phase were detected. However, phase composition, microstructure, and defect chemistry affected PL intensity and trapping. For sol-gel samples, PL was strongest at 1450 °C, whereas TSL was strongest at 1100 °C. Ca2+ and Sr2+ substitution reduced both signals and modified the glow-band shape, with Ca causing the larger decrease, consistent with dopant partitioning and charge compensation. Broad TSL may reflect distributed trap environments from mixed La/Gd/Y occupation. Solid-state samples showed stronger PL, higher TSL, and measurable UV-excited afterglow.