AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ perovskites: structural and redox properties

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• 作者：Porta ; Piero ; Cimino ; Stefano ; De Rossi ; Sergio ; Faticanti ; Marco ; Minelli ; Giuliano ; Pettiti ; Ida
• 关键词：Perovskite oxide solid solutions ; (La ; Sm ; Nd) iron perovskites ; La (Mg ; Fe) perovskites ; Fe⁴⁺-containing perovskites
• 刊名：Materials Chemistry and Physics
• 出版年：2001
• 期刊代码：58_02540584
• 类别：ms
• 出版时间：August 15, 2001
• 卷：71
• 期：2
• 页码：165-173
• 文件大小：218 K

摘要

The synthesis and properties of polycrystalline AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) perovskites prepared at different temperatures (623, 773, 923 and 1073K) 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 AFeO₃ 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 AFeO₃ perovskites, which, for charge balance, have small cation defectivity. The fraction of Fe⁴⁺ increases with the increase of x in LaFe_1−xMg_xO₃ 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 LaFe_1−xMg_xO₃ perovskites is due to the higher content of larger Fe³⁺ species formed by reduction of Fe⁴⁺. The decreasing reducibility of LaFe_1−xMg_xO₃ 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.