EPR and magnetization of La 2 NiO 4
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M. Vallet-Regi Instituto de Magnetismo Aplicado, Apdo. 155, Las Rozas, 28230 Madrid, Spain, and Departamento de Quimica Inorgdnica y Bioinorgdnica, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain
M. J. Sayagues Instituto de Magnetismo Aplicado, Apdo. 155, Las Rozas, 28230 Madrid, Spain, and Departamento de Quimica Inorgdnica, Facultad de Quimicas, Universidad Complutense, 28040 Madrid, Spain
R. D. Sanchez and M. T. Causa Centro Atomico Bariloche, 8400 San Carlos de Bariloche, Argentina (Received 29 March 1993; accepted 24 August 1993)
EPR and magnetization measurements of La2NiO4 samples prepared by two different methods are discussed. From the comparison with magnetization and EPR properties of metallic nickel it can be concluded that the sample prepared by the ceramic method is contaminated with ~1% of metallic nickel whereas no evidence of contamination is found in a sample prepared by the so-called liquid-mix technique.
I. INTRODUCTION Sometimes significant advances in materials science are registered, not as a result of the synthesis of new compounds, but rather as a consequence of refined variation in the processing of the material. The most common way to synthesize inorganic materials is the ceramic procedure, involving a solid-solid reaction, which usually takes place at high temperatures and long annealing times. Since the solid state reaction occurs first by a phase boundary reaction at the points of contact between the starting products, which are inhomogeneous at atomic level, and later by diffusion of the constituents through the product phase, the lack of homogeneity introduces kinetic constraints, decreasing the overall yield. Several modifications of the ceramic procedure have been used to avoid some of those limitations trying to decrease the diffusion path lengths. Materials prepared from precursors obtained by solution techniques with respect to those synthesized by the ceramic method can lead, in many cases, to important modifications of the physical properties. One of these methods involving molecular precursors, the so-called liquid mix technique,1 has been widely used to obtain perovskite-related mixed oxides with multiple occupation of the cationic sublattice. 23 Moreover, attempts to isolate the LaNiO 3 perovskite material by the ceramic method have been unsuccessful since at temperatures higher than 950 °C the La 2 NiO 4 phase is stabilized. However, the perovskite phase can be isolated at lower temperatures by using the liquid mix technique.4 176
J. Mater. Res., Vol. 9, No. 1, Jan 1994
La 2 Ni0 4+ 5 is now at the limelight due to the striking similarities with the superconducting ha2-xSTxCu04+s system. To obtain stoichiometric La 2 Ni0 4 a reduction process is necessary, sometimes leading to small amounts of metallic nickel.5 We describe, in this paper, an EPR study of La 2 NiO 4 prepared by two different processes, the ceramic method and the liquid mix technique. II. EXPERIMENTAL A. Sample preparation 1. Ceramic method Black oxidized La 2 Ni0 4+ g was synthesized
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