Synthesis, Crystallization, and Optical Properties of YAG:Cr3+ Aerogels

YAG:Cr³⁺ aerogels were prepared using a sol–gel method followed by supercritical CO₂ drying and annealing at 1000 °C to examine whether the well-known thermometric behavior of Cr³⁺ is preserved in a nanocrystalline and highly porous YAG structure. XRPD, SEM and TEM confirmed the formation of cubic YAG, with a lattice parameter of 12.065 Å and an average crystallite size of about 29 nm. Spectroscopic results also indicated that Cr³⁺ ions occupy the octahedral Al³⁺ sites in the YAG lattice. Temperature-dependent photoluminescence measurements were carried out from −150 to 600 °C and analyzed using a nine-Gaussian fitting model. At higher temperatures, the ⁴T₂/²E emission ratio followed Boltzmann behavior. For the 1 mol% Cr³⁺ sample, the fit from 30 to 150 °C gave ΔE = 890 ± 10 cm⁻¹ (R² = 0.999), in good agreement with the value of 917 cm⁻¹ obtained from lifetime measurements. At lower temperatures, where ⁴T₂ emission becomes weak, the anti-Stokes/Stokes phonon-sideband ratio was used instead. This method gave an effective phonon energy of 296 ± 6 cm⁻¹ (R² = 0.9962) between −150 and +20 °C. Together, the two approaches cover about 123–293 K and 303–423 K, with maximum relative sensitivities of 2.81% K⁻¹ and 1.39% K⁻¹, respectively. Among the tested Cr³⁺ concentrations, the 1 mol% sample showed the most reliable thermometric performance. Overall, the results show that YAG:Cr³⁺ retains its characteristic temperature-sensing behavior in aerogel form while also offering low density, high surface area and open porosity.