Synthesis of high surface area LaMnO3+d by a polymerizable complex method

Abstract

Powders of LaMnO3+d were synthesized by the Pechini-type polymerizable complex (PC) technique based upon in situ polyesterification between citric acid (CA) and ethylene glycol (EG). Heating at 90°C of a mixed solution of CA, EG, H2O, La and Mn ions with a molar ratio of CA/EG/H2O/La/Mn=1.5/4/20/0.02/0.02 gave a transparent polymeric resin without any precipitation. The processing temperature of 90°C was critical for obtaining a clear transparent gel; i.e. above 90°C hydrolysis of La- and/or Mn–CA species seems to be accelerated to form insoluble precipitates before the polymerization was completed. The polymerization itself was also crucial for precluding formation of insoluble precipitates; i.e. the non-polymerizable amorphous citrate route without EG was always accompanied with formation of insoluble precipitates during evaporation of water from CA/H2O solutions irrespective of the processing temperature. The polymeric resin or citrate-derived gel was preheated at 450°C to give a powder precursor for LaMnO3+d. XRD analyses indicated that better mixing of La and Mn cations was achieved in the PC-derived precursor than in the precursor derived from the non-polymerizable amorphous citrate route. The formation of pure perovskite LaMnO3+d occurred when the powder precursor was heat-treated in a static air at 700°C for 6 h. The LaMnO3+d powder prepared by the PC method had a considerably large surface area of 20.2 m2/g, when compared e.g. with 8.3 m2/g for the powder of the same compound derived from the more widely used coprecipitation technique.