Effect of phase separation in metal carboxylate gels on perovskite lead magnesium niobate crystallization

Phase separation during the synthesis and decomposition of lead magnesium niobate (PMN)-ethylenediaminetetraacetic acid (EDTA) and PMN-citrate gels strongly affects perovskite Pb(Mg_1/3Nb_2/3)O₃ phase formation. The PMN-EDTA gel was synthesized from a solution containing Pb-EDTA, Mg-EDTA, and peroxo-citrato-niobium complexes at pH 8. Pb-EDTA precipitation was avoided by using Pb:EDTA in the mole ratio >1:2.5 and flash pyrolyzing the PMN-EDTA solution at 225 °C. Consequently, the PMN yield increased from 80 to 98 wt%. The sequential decomposition of Pb-EDTA, Mg-EDTA, and peroxo-citrato-Nb in the PMN-EDTA precursor in 1 vol% O₂ leads to phase separation of Pb and PbO and thus lowers the PMN yield to 92 wt%. At P_O(2)>2.5 vol% the Pb, Mg, and Nb complexes cothermolyze to form ≥97% perovskite PMN. The presence of a heterometallic citrato-Pb-Mg-Nb complex in PMN-citrate leads to oxygen partial pressure independent codecomposition of the Pb, Mg, and Nb complexes. Accordingly, PMN yields of ≥96 wt% were obtained from the PMN-citrate precursor for oxygen partial pressures between 1 and 5 vol%.