Preparation of LaFeO 3 particles by sol-gel technology
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Preparation of LaFeO3 particles by sol-gel technology C. V´azquez-V´azquez,a) P. K¨ogerler, and M. A. L´opez-Quintela Department of Physical Chemistry, University of Santiago de Compostela, E-15706 Santiago de Compostela, Spain
R. D. S´anchez and J. Rivas Department of Applied Physics, University of Santiago de Compostela, E-15706 Santiago de Compostela, Spain (Received 12 February 1996; accepted 21 April 1997)
The study of submicroscopic particles in already known systems has resulted in a renewed interest due to the large differences found in their properties when the particle size is reduced, and because of possible new technological applications. In this work we report the preparation of LaFeO3 particles by the sol-gel route, starting from a solution of the corresponding metallic nitrates and using urea as gelificant agent. Gels were decomposed at 200 ±C and calcined 3 h at several temperatures, T , in the range 250–1000 ±C. The samples were structurally characterized by x-ray diffraction (XRD) showing that the orthoferrite crystallizes at T as low as 315 ±C. From the x-ray diffraction peak broadening, the particle size was determined. The size increases from 60 to 300 nm as the calcination T increases. Infrared spectroscopy was used to characterize gels and calcined samples. From these studies a mechanism for the gel formation is proposed. Study of the magnetic properties of LaFeO3 particles shows the presence of a ferromagnetic component which diminishes as the calcination temperature increases, vanishing at T 1000 ±C.
I. INTRODUCTION
Perovskite-type oxides containing transition metals have been known to show high catalytic activity for the complete oxidation of hydrocarbons, and their potentiality of replacing noble metals as combustion catalysts has been examined extensively.1,2 LaFeO3 is an orthoferrite which crystallizes in the space group D2h 16 -Pbnm3,4 and shows antiferromagnetic order below TN 740 K.5 This material is an orthorhombic distorted version of AMO3 cubic perovskites (A Sr, Ca, or a rare earth; M transition metal; e.g., SrFeO3 ) as a result of steric adjustments when smaller rare-earth elements are substituted at the A sites. The Fe atoms have an oxygen octahedral local environment and the distortion is manifested as a tilting of the octahedra off the c-axis direction. The degree of tilting is dependent on the size of the A atom and this determines the deviation of the Fe –O–Fe superexchange angle from 180±.6 Until now, almost all the studies about these compounds have been performed on single crystals and powders obtained by solid state reaction.7 This technique, which uses metal oxides as starting materials and needs several annealings at high temperatures during long periods of time with frequent intermediary grindings, has several problems, e.g., poor homogeneity a)
II. EXPERIMENTAL PROCEDURE
In the synthesis procedures carried out in this work all the reagent-grade chemicals employed were Aldrich
E-mail address: qfmatcvv@uscmai
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