Phase equilibria, crystal and magnetic structure of solid solutions LaFe_1-xCo_xO_3-d (0£x£1)

N.V. Proskurnina, V.I. Voronin

Institute of Metal Physics, Ural Division of Russian Academy of Sciences, S. Kovalevskoi Str.18, Ekaterinburg GSP-170, Russia

Perovskite-type oxides La_1-xA_xMeO₃ (A=Ba, Sr, Ca; Me- transition metal) have been extensively studied for over four decades due to their catalytic activities, unique magnetic, electrical and transport properties [1, 2] making these materials promising for different applications. In these compounds the rare earth element plays an important role to stabilize the structure, and the electronic ground state of 3d-transition metal (TM) ‘B’ is responsible for the magnetic and transport properties. The similarity of the structure to high-T_C superconductors and recent observation of giant magneto-resistance in hole doped LaMnO₃ have attracted a great interest in these compounds [3–5]. Therefore, manganites have been intensively investigated in recent years. At the same time Sr-, Ca- and Ba-doped cobaltites and ferrites can be used as gas-dense oxygen permeable ceramic membranes for gas conversation [6-8]. The possibility of creation of such devices is being studied nowadays. A number of scientific papers give much information about the phase equilibria and physical properties of solid solutions LnFe_1-xCo_xO₃, as a rule, when Ln = (La_0.3Sr_0.3) the structure is stabilized by Sr and therefore this solution exists in a continuous range of concentration [9]. However, no detailed structural LaFe_1-xCo_xO_3-d studies have been carried out.

The samples of the general composition LaFe_1-xCo_xO_3-d (0£x£1) with a step of 0.05 were prepared using different routes. First route was a traditional ceramic technique of three-stage firing in air in the temperature range of 850-1100°C for 100-300 hours with intermediate regrinding in ethanol. Another routes were preparation through amorphous precursors using nitric or citric acid as solvent. The final thermal treatment involved heating in air at 1100°C for about 150 hours. In order to identify phase composition all samples were examined by X-ray diffraction using a DRON-3 diffractometer with Cu-K_a radiation (10°£2q£75°). It is known that determination of the oxygen location in solid oxides by means of X-ray diffraction analysis lead to relatively large uncertainties. Therefore, neutron diffraction was used additionally for the crystal and magnetic structure determination. The powder neutron diffraction profiles were measured using neutron diffractometer D7a (wavelength of 1.5152 Е) at room temperature at the research reactor IVV-2, located near Yekaterinburg, Russia. For all single phase samples unit cells parameters and magnetic structure were calculated and refined by Rietveld analysis. Structural analysis was carried out by simultaneous refinement of neutron and X-ray powder diffraction data.

According to the X-ray analysis initial LaCoO_3-d has a cubic perovskite structure with rhombohedral distortions (space group R-3c). Rietveld neutron powder diffraction profile of this sample at room temperature is shown in figure 1. Single phase of the solid solution of following composition LaFe_1-xCo_xO_3-d was obtained in a range of concentrations 0£x£0.25. It is shown that the volume and unite cell parameters are continuously increase as the iron content increases. It caused by the ionic size difference: an ionic radius of Fe³⁺ is larger than ionic radius of Co³⁺.

Lanthanum ferrite LaFeO₃ was described in Pbnm space group with orthorhombic distortions [10]. All single phase samples LaCo_1‑yFe_yO_3-d exist within the composition range 0.775£y£1.0. In the samples in intermediate region both orthorhombic and rhombohedral boundary phases were simultaneously identified as LaCo_0.75Fe_0.25O₃ и LaCo_0.225Fe_0.775O₃.

It should be noted that single phase regions of LaCo_1‑yFe_yO_3-d are narrower than the regions of Sr-stabilized solutions La_0.7Sr_0.3Co_1‑yFe_yO_3-d [9].

In several reflexes some additional intensity of LaCo_0.1Fe_0.9O₃ was determined. It is known that the pure lanthanum ferrite is an antiferromagnetic (G-type) with Q_N= 750K [11-12]. Analysis of magnetic reflexes of experimental neutron diffraction profile of composition LaCo_1‑yFe_yO_3-d shows that antiferromagnetic G-type structure realize in this compound at room temperature (Fig.2). The magnitude of magnetic moment is 3.55(5) m_B on “average” iron atom in a unite cell.

All rhombohedral phases LaCo_1‑yFe_yO_3-d don’t have a magnetic moment at room temperature what is in a good agreement with data obtained for the LaCoO_3-d in [13-14].

Work supported by State Scientific Research Program “Topical Problems in Physics of Condensed Matter”, direction “Neutron Investigations of Condensed Matter” (State control No. 40.012.1.1.11.50), the basic research program of the Department of Physical Sciences of the Russian Academy of Sciences "Neutron studies of the substance structure and fundamental properties of matter" and Program for Support to Leading Scientific Schools of Russia (Project No. NS-639.2003.2).

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