Elsevier

Materials Research Bulletin

Volume 37, Issue 11, September 2002, Pages 1797-1813
Materials Research Bulletin

A study on the nuclear and magnetic structure of the double perovskites A2FeWO6 (A = Sr, Ba) by neutron powder diffraction and reverse Monte Carlo modeling

https://doi.org/10.1016/S0025-5408(02)00872-3Get rights and content

Abstract

Perovskite-type complex metal oxides A2FeWO6 (A=Sr, Ba) were prepared by a standard solid state reaction method. Rietveld analysis of neutron powder diffraction (NPD) data at 295 K shows that the Sr2FeWO6 (SFW) compound adopts a monoclinic unit cell (space group P21/n, a=5.6480(4), b=5.6088(4), c=7.9362(6) Å and β=89.99(2)°), and the Ba2FeWO6 (BFW) compound is tetragonal (space group I4/m, a=5.7547(4), c=8.125(1) Å). A combination of a reverse Monte Carlo (RMC) technique and Rietveld analysis shows that the low temperature (10 K) magnetic structures of SFW and BFW are antiferromagnetic based on a unit cell related to that of the nuclear structure by a propagation vector, k=(01/21/2). The magnetic moment of iron was found to be 3.86(4)μB and 3.49(2)μB at 10 K for the Sr- and the Ba-containing compounds, respectively.

Introduction

Recently research on double perovskites has gained strong interest in particular after the discovery of the metallic and ferrimagnetic characteristic in Sr2FeMoO6 and Sr2FeReO6 by Kobayashi et al. [1]. The compounds show a critical temperature, Tc around 420 and 400 K, respectively. Since half-metallic ferromagnetism and magnetoresistance (MR) seem to be intimately related to each other, there is an intensive search for half-metallic ferromagnets, which could be candidate materials for the realisation of MR applications. The modification of structural and magnetic properties by changing the A, B′ and/or B″-site cations in A2B′B″O6 type double perovskites has also gained interest in order to better understand the mechanism of colossal magnetoresistance (CMR) and other unusual physico-chemical properties. Experimental and theoretical efforts have now established a strong coupling of structural, magnetic and electronic property degrees of freedom. Many researchers [1], [2], [3], [4], [5], [6] have reported these properties of the double perovskite transition metal oxides, especially for the Fe-based compounds. The structure and physical properties of this type of double perovskites depend strongly on the size and valences of the A, B′ and B″-site cations. Ordering of the B-site cations in the octahedral sublattice [7] plays an important rule on the half-metallic characteristics of the compounds.

SFW was first prepared and characterized as cubic with a=7.96 Å by Blasse [8]. Later Nakagawa et al. [9] reported the compound as tetragonal at room temperature with a=7.925(1) and c=7.985(1) Å. Kawanaka et al. [3], [4] has found a smaller tetragonal cell with the lattice parameters a=5.605 and c=7.947 Å and the antiferromagnetic (AFM) transition temperature TN=37 K. The compound BFW was first reported to be a cubic with a≈8.133 Å [5], [10]. All above-mentioned structural studies were done by X-ray powder diffraction (XPD). Recently, we have reported the structure of BFW as tetragonal based on neutron powder diffraction (NPD) [11]. From a structural point of view, our result differs from the ones found in the previous studies. It may be that NPD is a more accurate method to determine the positions of the atoms, especially for light atoms in complex metal oxides, than XPD. We have collected temperature-dependent NPD data in order to extract information concerning the atomic and magnetic structure.

For perovskite systems, the valence of Fe is 3+ in most of the compounds. A new device has already been proposed using organic spin-transition polymers, in which Fe2+ ion cause a low-spin–high-spin transition [12]. The double perovskite oxide with iron might be another candidate for spin-transition compounds with Fe2+ ions [3], [4]. We used a bond valence sum calculation to predict the oxidation-state of Fe and W, which shows mixed valence states for them. We have prepared the compounds in single-phase form and elucidated the nuclear and magnetic structures by using X-ray and temperature-dependent neutron diffraction. The nuclear and magnetic structure is discussed using information from Rietveld analysis and RMC modeling. Structural parameters calculated from the diffraction data are very important in order to understand the physical properties of the compounds since they have a strong correlation. Both the compounds are very interesting to study, because the closely related compounds, such as Sr2FeMoO6 [1], [13] and Ba2FeMoO6 [2], [13], show CMR properties at high temperature.

Section snippets

Sample preparation

Polycrystalline samples of A2FeWO6 (A=Sr, Ba) were prepared by the conventional ceramic method from stoichiometric amounts of high purity (99.95% or above) starting materials: SrCO3 (Mallinckrodt Chemical Works, USA), BaCO3 (Heraeus, Germany), Fe2O3 (Fischer Chemical, USA) and WO3 (Chemicon, Sweden). The color and formula weight of the starting materials were: SrCO3—white, 147.63 g/mol; BaCO3—white, 197.35 g/mol; Fe2O3—red, 159.69 g/mol; and WO3—light green, 231.85 g/mol. The powders were carefully

Nuclear structure of SFW

The NPD pattern of SFW could be indexed in space group P21/n with a=5.6480(4), b=5.6088(4) Å, c=7.9362(6) Å and β=89.99(2)° at 295 K. Other space groups, in particular tetragonal and orthorhombic ones, were tried but the refinement was not satisfactory. A small difference in a and b lattice parameters and a monoclinic β angle close to 90° indicates that the metric of this structure seems to be strongly pseudo-orthorhombic or even pseudo-tetragonal. The real driving force for the monoclinic

Conclusions

In summary, we have presented a detailed crystallographic and magnetic characterization of the double perovskites A2FeWO6 (A=Sr, Ba) by means of temperature-dependent neutron diffraction measurements. Our results are consistent with an antiferromagnetic coupling of the magnetic moments caused by the unpaired electrons at the iron sites in both of the compounds at low temperature. No evidence of significant mis-site cation disorder was found during refinements of neutron diffraction data. There

Acknowledgements

We would like to acknowledge the financial support from the Swedish Natural Science Research Council (NFR), The Royal Academy of Sciences and the Swedish Foundation of Strategic Research (SSF). A.K. Azad gratefully acknowledges the financial support from the “Research, development and training project of the Bangladesh Atomic Energy Commission.”

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