Identification of Phase Evolution through Upconversion Emission in Flame-Made YAG:Yb³⁺/Er³⁺ Nanoparticles: Implications for the Optical Crystal-Phase Probe

High-phase purity yttrium aluminum garnet (YAG) nanoparticles (NPs), which account for more than half of the market share in solid-state lasers, are crucial luminescent ceramic matrixes used in upconversion luminescence (UCL) applications. As is well known, the optical performance of YAG NPs is greatly influenced by their phase purity, necessitating a long-duration, high-temperature annealing process to eliminate intermediate impurity phases and subsequent inspection of the crystal phases using traditional methods such as XRD. Here, we report a convenient direct method to evaluate the phase state of YAG NPs by correlating the macroscopic UCL properties with the phase evolution process of YAG NPs, given the strong dependence of UCL on the host phase structure. Yb 3+ /Er 3+ -codoped YAG NPs were fabricated using flame spray pyrolysis (FSP), an efficient industrial mass production method. The UCL intensity ratios of 556 and 546 nm ( I 556 / I 546 ) for Er 3+ ions are used as a key parameter to assess the YAG phase states. When the I 556 / I 546 ratio is larger than 1.80, FSP-prepared YAG:Yb 3+ /Er 3+ NPs undergo a complete phase transformation from amorphous to hexagonal YAlO 3 (YAH) and ultimately to cubic YAG, resulting in pure cubic YAG NPs with orange-to-green UCL color modulation. Our results offer an efficient optical crystal-phase probe applied for YAG and can enable the exploration of industrial phase monitoring for other optical materials.