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 PO₂ > 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%.