To suppress the thermal decomposition and stabilize the crystal structure of Gd3Al5O12 (GdAG) garnet, GdAG was doped with significantly smaller Lu3+ to form (Gd,Lu)AG solid solutions. The precursors for (Gd1-xLux)3AG (x = 00.5), with a general formula of (NH4)xLn3Al5(OH)y(CO3)z.nH2O (Ln = Gd and Lu), were synthesized via carbonate co-precipitation from the mixed solutions of aluminum, lutetium, and gadolinium nitrates using ammonium hydrogen carbonate as the precipitant. The results showed that 10 at.% of Lu3+ (x = 0.1) has been able to stabilize GdAG against its thermal decomposition to a mixture of GdAlO3 (GdAP) and Al2O3 phases at temperatures above 1300 degrees C and that Lu3+ doping effectively lowers the temperature for garnet crystallization. The precursors of x = 0.3 convert to pure LnAG at a low temperature of 1000 degrees C via the intermediates of Ln4Al2O9 (LnAM) and LnAP. The carbonate precursors are loosely agglomerated and the resultant LnAG powders show good dispersion and a fairly uniform particle morphology. The (Gd,Lu)AG solid solutions exhibit decreasing lattice parameters and increasing optical bandgaps along with more Lu3+ incorporation.