Microstructure and Optical Properties of Y1.8La0.2O3 Transparent Ceramics Prepared by Spark Plasma Sintering

Yttrium oxide ceramic is an excellent optical material widely used in lasers, scintillators, and upconversion luminescence. In this study, LiF was employed as an additive to generate volatile gases (CF)n to effectively inhibit carbon contamination and act as a sintering aid to accelerate densification during the spark plasma sintering (SPS) process. The effects of sintering temperature and annealing time on the transmittance of Y1.8La0.2O3 transparent ceramics were systematically investigated. Results indicate that excessive LiF addition reduces the transmittance of Y1.8La0.2O3 transparent ceramics due to the precipitation of F− ions at grain boundaries, forming a secondary phase. For the Y1.8La0.2O3 ceramics with 0.3 wt.% LiF, transmittance initially increases and then decreases with rising sintering temperature, reaching a maximum value of 78.10% in the UV region at 1550 °C. Under these conditions, the average particle size and relative density are 10–30 μm and 99.36%, respectively. Oxygen vacancies within the ceramics act as defects that degrade transmittance. Proper annealing in air reduces oxygen vacancy content, thereby improving transmittance. After annealing at 900 °C for 3 h, the maximum transmittance of Y1.8La0.2O3 ceramics with 0.3 wt.% LiF increases to 82.67% in the UV region, accompanied by a 5.68% reduction in oxygen vacancy concentration.