(Gd1-xYx)(2)O2CO3:y% Tb3+ (0 < x < 0.3, 7 < y < 17) phosphor powders of submicrometer particle size were synthesized using an original flux method with a 0.476Li(2)CO(3)-0.270Na(2)CO(3)-0.254K(2)CO(3) eutectic mixture. The crystal field around the Tb3+ ions was controlled by doping Y3+ ions into the Gd2O2CO3:Tb3+ lattice, which resulted in a superior emission intensity compared to that of our previously developed Gd2O2CO3:15% Tb3+ phosphor, of which the relative emission intensity was 111% that of a commercially available LaPO4:Ce3+,Tb3+ phosphor. The highest emission intensity was successfully obtained by optimizing the composition of the (Gd,Y)(2)O2CO3:Tb3+ phosphor while maintaining a submicron particle size. The relative green emission reached a maximum intensity for the (Gd0.95Y0.05)(2)O2CO3:11% Tb3+ composition, which was 131% that of LaPO4:Ce3+,Tb3+. By increasing the crystallinity of the phosphor as well as the crystal field around Tb3+, the fluorescence quantum yield of the terbium emission for this phosphor became 75%, which is significantly higher than that of Gd2O2CO3:15% Tb3+ (55%).