AFeO3 (A = La, Nd, Sm) and LaFe1-xMgxO3 perovskites:: structural and redox properties

The synthesis and properties of polycrystalline AFeO(3) (A = La, Nd, Sm) and LaFe1-xMgxO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) perovskites prepared at different temperatures (623, 773, 923 and 1073 K) from decomposition of citrate precursors are reported. Chemical analysis, X-ray diffraction (XRD) for phase analysis, lattice parameters and crystallite dimension, BET surface area determination, diffuse reflectance spectroscopy, and magnetic susceptibility measurements were employed for the characterization of the solids. The reducibility of the samples was followed by temperature programmed reduction (TPR) and by reduction with in situ XRD. The calcination at different temperatures revealed that samples having less disordered perovskite. structure are more easily formed. The variation of lattice constants and the order of reducibility within the AFeO(3) perovskites, LaFeO3 > NdFeO3 > SmFeO3, have been linked to different Fe-O bond strength. A certain fraction of Fe4+ was observed by redox. titration and TPR to be present in AFeO(3) perovskites, which, for charge balance, have small cation defectivity. The fraction of Fe4+ increases with the increase of x in LaFe1-xMgxO3 solid solutions. However, the Fe4+/Mg2+ ratio never reaches unity, so that these mixed perovskites, to preserve charge neutrality, contain oxygen defectivity. After reduction, all samples preserve the perovskite structure and the expansion of the unit cell volume in the reduced LaFe1-xMgxO3 perovskites is due to the higher content of larger Fe3+ species formed by reduction of Fe4+. The decreasing reducibility of LaFe1-xMxO3 solid solutions at the increase of x has been correlated with oxygen atoms more strongly bonded. Optical and magnetic properties are also reported and discussed. (C) 2001 Elsevier Science B.V. All rights reserved.