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By pre-treating and component design of the ceramic powders, cubic (Lu,Gd) 2 O 3 :Eu solid-solution ceramics with high optical quality were achieved by the solid-state reaction method combined with vacuum sintering. The effects of Gd3+ doping (x = 0.05–0.6) on powder characteristics, phase compositions, microstructures and optical performances of the (Lu 0.95- x Gd x Eu 0.05) 2 O 3 ceramics were systematically investigated to obtain the optimum Gd3+ content. The results indicated that the incorporation of moderate amounts of Gd3+ (5–50 mol%) effectively promoted mass diffusion and grain growth during sintering. The 50 mol% Gd3+-doped Lu 2 O 3 :Eu transparent ceramic (using the powder calcined at 1200 °C) sintered at 1750 °C for 5 h exhibited the optimum transmittance of 77.0 % in the visible spectral region (∼1 mm in thickness), which was significantly higher than the reported transmittance of vacuum sintered (Lu,Gd) 2 O 3 :Eu ceramics (less than 65.0 %). Owing to the lower electronegativity of Gd3+ relative to Lu3+, the bandgap energies of the final ceramics decreased gradually with increasing Gd3+ substitution, and the redshift of the CTB centre was observed with more Gd3+ addition. Under CTB excitation, the incorporation of Gd3+ enhanced the red emission intensity and shortened the fluorescence lifetime. • Highly transparent (Lu,Gd) 2 O 3 :Eu ceramics were achieved by pre-treating and component design of ceramic powders. • Gd3+ doping promotes mass diffusion and grain growth during sintering. • The (Lu 0.45 Gd 0.5 Eu 0.05) 2 O 3 ceramic exhibits the transmittance of 77.0 % in the visible range. • Incorporation of Gd3+ results in enhanced luminescence intensity and shortened lifetime. [ABSTRACT FROM AUTHOR]