Diffraction and spectroscopic study of pyrochlores Bi2−xFe1+xSbO7

doi:10.1016/j.jallcom.2013.11.226

Journal of Alloys and Compounds

Volume 589, 15 March 2014, Pages 425-430

https://doi.org/10.1016/j.jallcom.2013.11.226 Get rights and content

Highlights

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Fe rich pyrochlores of the type Bi₂₋_xFe_1+xSbO₇ were prepared by solid state methods.
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Structures determined using a combination of neutron and synchrotron X-ray diffraction.
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Fe partially occupies the 8-coordinate site.
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Dispacive disorder of the Bi cations observed as a consequence of the 6s² electrons.
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Non-Vegard behaviour seen at low Fe contents due to disorder.

Abstract

The structural and electronic properties of the series Bi₂₋_xFe₁₊_xSbO₇ (0 ⩽ x ⩽ 0.6) were investigated using a combination of diffraction and spectroscopy. Synchrotron and neutron diffraction analysis show that Fe³⁺ cations substitute for Bi³⁺ onto the A site with increasing x, which was further confirmed by analysis of the Fe K/L-edge X-ray absorption near-edge spectra. The diffraction analysis indicated the presence of displacive disorder along the A₂O chains, likely the result of the Bi³⁺ 6s² lone pair, as well as non-Vegard-like behaviour of the lattice parameters in the Fe-poor region. Fe K-edge extended X-ray absorption fine-structure analysis of Bi₂FeSbO₇ confirmed the displacive disorder of the Bi³⁺ cations as well as Sb⁵⁺ and Fe³⁺ disorder on the B site.

Introduction

Extensive efforts have been made in recent years to develop bismuth-based pyrochlores for applications including high-frequency multilayer capacitors [1], [2], catalysts [3], [4], thin film resistors [5], as well as to understand their interesting structural and physical properties [6], [7]. Pyrochlores are also of interest as hosts for immobilisation of radioactive waste [8] and for their low thermal conductivity [9]. A number of their interesting properties are related to the flexibility of the pyrochlore structure. The ideal A₂B₂O₆O′ pyrochlore structure (space group $Fd \bar{3} m$ ) is derived from a fluorite supercell (a_p = 2a_f) with oxygen atoms ordered around the A and B cations [10]. The larger A cations occupy the 16d site (½, ½, ½) and the smaller B cations the 16c (0, 0, 0) site. The anions occupy two sites, with O on the 48f site (x, 1/8, 1/8) and O′ the 8b site (3/8, 3/8, 3/8). An alternative description of the unit cell has the larger A cation occupying the 16c site and the O′ anion the nominally vacant 8a site. The structure can be described as an interpenetrating network of corner-sharing BO₆ octahedra and A₂O′ chains [10]. The A-site cations are actually eight-coordinate with two short A–O′ and six long A–O bonds, forming a compressed scalenohedral coordination. Pyrochlores that contain lone pair active cations on the A site can often exhibit static displacement disorder, a phenomenon where the lone pair electrons on the A-site cation causes distortions along the A₂O chain [11]. Such displacements have been observed is several Bi³⁺ containing pyrochlores (e.g., Bi₂InNbO₇, Bi₂₋_xM_xRu₂O₇ and Bi₂₋_xLn_xTi₂O₇) due to the presence of the 6s² lone pair [11], [12], [13]. Static displacement disorder has also been suggested to occur in pyrochlores containing other lone-pair active cations, such as Pb²⁺ (e.g., Pb₂Sn₂O₇₋_δ, Pb₂Ir₂O₇₋_δ) [14], [15] and Sn²⁺ (Sn₂Nb₂O₇) [16]. Disorder in the A₂O′ chain occurs as the A cations are displaced from the 16d to the 96h site (0, y, −y), which forces the O′ anion from the 8b site to the 32e site (x, x, x) [12], [13], [17]. This results in six-fold disorder of the A cation and the four-fold disorder of O′ anions (Fig. 1) [13].

Recently it has been reported by Luan and Hu that Fe₂BiSbO₇ forms an ordered pyrochlore structure with Fe³⁺ fully occupying the A site and Bi³⁺/Sb⁵⁺ occupying the B site [18]. Such an arrangement is rather surprising since Bi³⁺ (1.17 Ǻ) is much larger than Fe³⁺ (0.78 Ǻ) in 8-fold coordination [19] and on size arguments the Bi³⁺ should preferentially occupy the larger 8-coordinate sites. This arrangement was observed for Bi₂FeSbO₇, which was originally reported to be an ordered pyrochlore with Fe³⁺ occupying only the B site [20]. A recent diffraction and Mössbauer spectroscopy study by Whitaker et al. [21] and Egorysheva et al. [22] suggested that Fe³⁺ can partially occupy the A site in Fe-doped Bi₂₋_xFe₁₊_xSbO₇, which can written as (Bi₁₋_xFe_x)₂(FeSb)₂O₇, up to x = 0.3. However these authors found no evidence of Bi³⁺ occupying the B site. Although it is unusual for transition-metal ions to occupy the A site, static displacement disorder can stabilize the substitution of small cations onto the A site, as evident from diffraction studies on Bi_1.5Zn_0.92Nb_1.5O_6.92 and Bi₂₋_xCu_xRu₂O₇₋_δ [12], [23]. In principle, Fe³⁺ could occupy the A sites in Bi₂₋_xFe₁₊_xSbO₇ as a consequence of the displacement of the Bi³⁺/Fe³⁺ and O′ ions from their ideal positions [21]. To clarify some of the inconsistencies in the structural analysis of the Bi₂FeSbO₇ and Fe₂BiSbO₇, we describe here the results of our studies of the crystal and electronic structure of a number of members of the Bi₂₋_xFe₁₊_xSbO₇ solid solution. The structure of Bi₂₋_xFe₁₊_xSbO₇ was investigated using synchrotron X-ray and neutron powder diffraction, while X-ray absorption spectroscopy (XAS) was employed to determine the oxidation states of the Fe and Bi cations.

Section snippets

Synthesis

Polycrystalline samples Bi₂₋_xFe₁₊_xSbO₇ where x = 0.00, 0.10 … 1.0 were prepared from stoichiometric mixtures of Bi₂O₃ (Sigma–Aldrich, ⩾99.9%), Sb₂O₅ (Sigma–Aldrich, 99.995%) and Fe₂O₃ (Sigma–Aldrich 99.98%). The starting reagents were mixed as an acetone slurry in an agate mortar and pestle, pressed into pellets and then heated at 650 °C for 24 h and 900 °C for 96 h with intermittent regrinding and re-pressing.

Diffraction analysis

Neutron powder diffraction data were obtained for selected samples using the high resolution

Diffraction studies

Portions of the synchrotron X-ray diffraction profiles for members of the series Bi₂₋_xFe₁₊_xSbO₇ are illustrated in Fig. 2 and demonstrate the presence of a pyrochlore phase in all cases. Examination of the profiles shows a number of additional reflections appear in samples with x ⩾ 0.5, indicating this is the limit of solubility of Fe. Our results are similar to that described by Whitaker et al. [21], leading us to conclude that the earlier report of Fe₂BiSbO₇ is incorrect [18] The lattice

Conclusion

The results of our diffraction and spectroscopic investigation of the structure of Bi₂₋_xFe₁₊_xSbO₇ are in good agreement with the results reported by Whitaker et al. [21] and Egorysheva et al. [22] Synchrotron and neutron diffraction analysis confirmed that Fe³⁺ cations occupy both the A and B sites when x > 0. Despite previous reports of Bi³⁺ cations occupying the B site in Fe₂BiSbO₇ [18], there was no evidence of extensive Bi³⁺ cation site disorder in Bi₂₋_xFe₁₊_xSbO₇. However, diffraction and

Acknowledgments

We acknowledge the support of the Australian Synchrotron for this work which was, in part, performed at the Soft-X-ray and powder diffraction beamlines at the Australian Synchrotron and at the Australian National Beamline Facility (ANBF) at the Photon Factory in Japan. We acknowledge the Australian Research Council for financial support and the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Japan, for operations support of the ANBF.

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Diffraction and spectroscopic study of pyrochlores Bi2−xFe1+xSbO7

Highlights

Abstract

Introduction

Section snippets

Synthesis

Diffraction analysis

Diffraction studies

Conclusion

Acknowledgments

Mater. Res. Bull.

Chem. Phys. Lett.

J. Solid State Chem.

Prog. Solid State Chem.

J. Solid State Chem.

J. Solid State Chem.

J. Solid State Chem.

Solid State Sci.

J. Solid State Chem.

J. Solid State Chem.

Solid State Commun.

J. Solid State Chem.

J. Alloys Comp.

J. Solid State Chem.

Physica B

J. Solid State Chem.

J. Alloys Comp.

Appl. Phys. Lett.

J. Carbohydr. Chem.

J. Appl. Phys.

Chem. Mat.

J. Phys. Chem. B

Diffraction and spectroscopic study of pyrochlores Bi₂₋_xFe₁₊_xSbO₇