Structure and transport in rare-earth ferrates Andrey Berenov ⁎ , Eloisa Angeles, Jeremy Rossiny, Edwin Raj, John Kilner, Alan Atkinson Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, UK Received 11 July 2007; received in revised form 10 January 2008; accepted 20 January 2008 Abstract The Rare-Earth ferrates REFeO 3 (RE = La, Pr, Sm, Gd, Ho) were prepared by the citric acid route. The distortion in the FeO 6 octahedron decreased with the decrease in the ionic radii (except for Ho) and the angle between adjacent FeO 6 octahedra decreased with the ionic radius of the rare earth. Electrical conductivity showed semiconducting behaviour with a complex dependence of the activation energy on the ionic radii of RE ion. Fast oxygen desorption was observed above 300 °C. No effect of A-site cation on the coefficients of thermal expansion was observed. © 2008 Elsevier B.V. All rights reserved. Keywords: Cation disorder; Perovskite; Electrical conductivity; Crystal structure 1. Introduction Solid Oxide Fuel Cells operating at intermediate tempera- tures (around 600 °C) attract considerable attention because they require less expensive materials and promise enhanced durability [1], however, lower temperatures lead to slower electrode kinetics and hence new cathode materials are needed that exhibit enhanced oxygen diffusion at lower temperatures. Undoped and Sr doped lanthanum ferrates, La 1 - x Sr x FeO 3 , have been extensively studied in the past [2,3]. The ferrates showed semiconducting behavior with a transition from p-type conductivity at Po 2 N 10 - 4 atm to n-type conduction at Po 2 b 10 - 12 atm [3]. Recently, they have attracted renewed interest due to the promising cathodic performance of the mixed cobalt ferrate Ba 1 - x Sr x Co 1 - y Fe y O 3 - d perovskites [4]. However very limited data exists on the effect of the A-site Rare-Earth (RE) ion on the crystal structure and transport properties in Fe-based ceramics. In this paper the crystal structure and transport properties of Rare- Earth ferrates with the perovskite structure are studied. 2. Experimental Rare-earth ferrates REFeO 3 (RE =La, Pr, Sm, Gd, Ho) were prepared by the citric acid route. Rare-earth oxides: La 2 O 3 (Alfa Aesar, REacton, 99.99%), Pr 6 O 11 (Alfa Aesar, REacton 99.9%), Sm 2 O 3 (Acros Organics, 99.9%), Gd 2 O 3 (Sigma-Aldrich, 99.9%), Ho 2 O 3 (Sigma-Aldrich, 99.9%), were precalcined at 1000 °C overnight. Stoichiometric amounts of the rare-earth oxides and Fe (Alfa Aesar, 99.9+%) were dissolved in nitric acid. Citric acid was added to the solution until the citric ion to the total metal ion ratio was 1:1.1. The resulting gels were combusted and annealed at 500 °C for 5 h in air. The solids were further annealed at 1000 °C for 10 h with intermediate grinding. Rectangular bars were isostatically pressed at 300 MPa and sintered at 1400 °C for 2 h in air. The density of sintered specimens was measured by the Archimedes method. Powder X-ray diffraction, XRD, patterns were recorded on a Philips 1710 diffractometer using monochromatic Cu-Kα radiation. Si was used as an external standard. The Rietveld method [5] was used to refine the XRD data. The XRD peak shape was convoluted to pseudo-Voigt function [6], and the background was refined to a fifth-degree polynomial. In the final refinement the following parameters were refined: five background coefficients, twelve peak shape parameters, zero point, scale factors, positional, isotropic thermal factors and unit-cell parameters. Site occupancies were fixed according to the results of chemical analysis. Cation ratios were measured using an inductively coupled plasma atomic emission spectro- scopy (ICP-AES, Varian Vista Pro) and shown in Table 1. Traces of Ca, Mg, Na, Co were detected at ppm levels in the powders. Available online at www.sciencedirect.com Solid State Ionics 179 (2008) 1090 – 1093 www.elsevier.com/locate/ssi ⁎ Corresponding author. E-mail address: a.berenov@imperial.ac.uk (A. Berenov). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.025