PEROVSKITE-TYPE OXIDES: I. STRUCTURAL, MAGNETIC, AND MORPHOLOGICAL PROPERTIES OF LaMn1-xCuxO AND LaCo1-xCuxO3 SOLID SOLUTIONS WITH LARGE SURFACE AREA

摘要

Perovskite-type compounds of general formula LaMn1-xCuxO3 and LaCo1-xCuxO3 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by calcining the citrate gel precursors at 823, 923, and 1073 K. The decomposition of the precursors was followed by thermal analysis and the oxides were investigated by means of elemental analysis, powder XRD, BET surface area, X-ray absorption (EXAFS and XANES), electron microscopy (SEM and TEM), and magnetic susceptibility. LaMn1-xCuxO3 samples are perovskite-like single phases up to x=0.6. At x=0.8 CuO and La2CuO4 phases are present in addition to perovskite. For x=1.0 the material is formed by CuO and La2CuO4. Mn(IV) was found by redox titration in all Mn-based perovskite samples, its fraction increasing with the increase in Cu content. EXAFS and XANES analyses confirmed the presence of Mn(IV). Cation vacancies in equal amounts in the 12-coordinated A and octahedral B sites are suggested in the samples with x=0.0 and x=0.2, while for x=0.6 anionic vacancies are present. Materials with sufficiently high surface area (22-36 m2 g-1 for samples fired at 923 K and 14-22 m2 g-1 for those fired at 1073 K) were obtained. Crystallite sizes in the ranges 390-500 and 590-940 angstroms for samples calcined at 923 and 1073 K, respp. , were determined from the FWHM of the (102) XRD peak. TEM patterns of LaMnO3 showed almost regular hexagonal prismatic crystals with sizes of the same order of magnitude (800 angstroms) of those drawn from XRD, while no evidence of defect clustering was drawn out from TEM and electron diffraction images. For the sample with x=0.6, TEM and electron diffraction patterns revealed perturbation of the structure. Magnetic susceptibility studies show a ferromagnetic behavior that decreases with increase in x. LaCo1-xCuxO3 samples are perovskite-like single phases up to x=0.2. For x=0.4 a small amount of La2CuO4, in addition to perovskite, is detected. For x greater than or equal to 0.6 massive formation of La2CuO4 and CuO is observed. Only trivalent Co is found by redox titration. Magnetic susceptibility studies have shown that trivalent Co is present in all samples as a mixture of paramagnetic Co3+ and diamagnetic CoIII ions, the Co3+ fraction being, at least up to x=0.4, approx equal to 0.34. Antiferromagnetic behavior, which increases with increase in x, is observed in all LaCo1-xCuxO3 samples. LaCoO3 is a stoichiometric perovskite. The substitution of Co by Cu2+ leads to a positive charge defectivity which is compensated by O vacancies. EXAFS and XANES analyses confirmed the presence of trivalent Co. Materials with fairly high surface area (in the ranges 19-27 and 13-21 m2 g-1 for samples calcined at 923 and 1073 K, respp. ) were obtained. Crystallite sizes of approx equal to 400 and approx equal to 1000 angstroms for samples calcined at 923 and 1073 K, respp. , were determined from the FWHM of the (102) XRD peak. Materials with not very clear morphology and crystals with definite structure are distinguishable by SEM for samples calcined at 1073 and at 1273 K, respp. TEM patterns, for samples calcined at 1073 K, evidence almost regular hexagonal prismatic crystals connected to form "linked structures" and some "spotty crystals," suggesting short-range ordered local defects. For Cu-containing samples, calcined at 1273 K, a higher degree of defectivity and the occurrence of "planar faults" are shown by TEM. 33 refs.