Article pubs.acs.org/IC
Topochemical Reduction of the Ruddlesden−Popper Phases Sr2Fe0.5Ru0.5O4 and Sr3(Fe0.5Ru0.5)2O7 Fabio Denis Romero,† Diego Gianolio,‡ Giannantonio Cibin,‡ Paul A. Bingham,§ Jeanne-Clotilde d’Hollander,§ Susan D. Forder,§ and Michael A. Hayward*,† †
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom ‡ Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom § Materials and Engineering Research Institute, Faculty of Arts, Computing, Engineering and Sciences, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, United Kingdom S Supporting Information *
ABSTRACT: Reaction of the Ruddlesden−Popper phases Sr2Fe0.5Ru0.5O4 and Sr3(Fe0.5Ru0.5)2O7 with CaH2 results in the topochemical deintercalation of oxide ions from these materials and the formation of samples with average compositions of Sr2Fe0.5Ru0.5O3.35 and Sr3(Fe0.5Ru0.5)2O5.68, respectively. Diffraction data reveal that both the n = 1 and n = 2 samples consist of two-phase mixtures of reduced phases with subtly different oxygen contents. The separation of samples into two phases upon reduction is discussed on the basis of a short-range inhomogeneous distribution of iron and ruthenium in the starting materials. X-ray absorption data and Mössbauer spectra reveal the reduced samples contain an Fe3+ and Ru2+/3+ oxidation state combination, which is unexpected considering the Fe3+/Fe2+ and Ru3+/Ru2+ redox potentials, suggesting that the local coordination geometry of the transition metal sites helps to stabilize the Ru2+ centers. Fitted Mössbauer spectra of both the n = 1 and n = 2 samples are consistent with the presence of Fe3+ cations in square planar coordination sites. Magnetization data of both materials are consistent with spin glass-like behavior.
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INTRODUCTION The wide variety of electronic and magnetic behavior exhibited by complex transition metal oxides has motivated an intense and enduring interest in these materials. The diverse array of physical properties exhibited by these systems can be attributed to the presence of electrons in partially occupied metal d orbitals and/or states that are coupled to each other via exchange and other orbital interactions, to yield cooperative, correlated behavior.1 As a consequence, the electronic structures of complex transition metal oxides depend strongly on the transition metal local coordination geometry and oxidation state, which together define the local transition metal electronic configuration and the connectivity of the extended metal oxide lattice, which defines the nature of the coupling interactions between metal centers. Given the decisive influence these parameters have on determining the physical properties of complex oxides, an effective strategy for discovering new materials that exhibit novel correlated electronic behavior is the preparation of phases that contain transition metal cations in unusual oxidation states and/or coordination geometries embedded within novel metal−oxygen networks. Conventional high-temperature synthesis routes allow the preparation of a wide range of complex oxide phases. However, © 2013 American Chemical Society
although this method is widely employed, it is restricted to preparing only the most thermodynamically stable phases in any composition range, limiting the variety of transition metal oxidation states and coordination geometries that can be realized in the materials formed in this way. In contrast, lowtemperature “soft” chemical approaches allow a degree of kinetic control to be applied in formation of reaction products. This allows the preparation of nonequilibrium, metastable phases, extending the range of materials that can be prepared. Using this approach, binary metal hydrides (NaH, LiH, and CaH2) have been employed as solid state reducing agents to effect the low-temperature, topochemical reduction of complex, first-row (3d) transition metal oxides.2−4 This has allowed the preparation of phases with highly unusual transition metal oxidation states and metal coordination geometries, such as square-planar Ni+, Co+, Co2+, and Fe2+ and even octahedral Mn+.2,5−8 Recently, our attention has turned to complex oxides containing second- and third-row (4d and 5d) transition metals. Initial investigations indicated that under the action of Received: April 15, 2013 Published: September 16, 2013 10920
dx.doi.org/10.1021/ic400930y | Inorg. Chem. 2013, 52, 10920−10928
Inorganic Chemistry
Article
air at 1200 °C for 2 days. Samples were then reground and heated for two further periods of 2 days at 1350 °C. As a final step, samples of Sr3Ru2O7 were quenched from high temperature into liquid nitrogen to avoid partial decomposition into Sr2RuO4 and SrRuO3. X-ray powder diffraction data confirmed the formation of single-phase samples with lattice parameters in good agreement with literature values.13,14 Reactions between Srn+1RunO3n+1 (n = 1 or 2) phases and 2n molar equivalents of CaH2 were performed in sealed silica tubes. At temperatures below 400 °C, no reaction was observed. At temperatures above 400 °C, reactions of both Sr2RuO4 and Sr3Ru2O7 resulted in decomposition to form mixtures of SrO, CaO, and elemental ruthenium. Characterization. X-ray powder diffraction data were collected from samples contained in gastight sample holders using a PANalytical X’Pert diffractometer incorporating an X’celerator position sensitive detector (monochromatic Cu Kα1 radiation). Neutron powder diffraction data were collected using the D2B instrument (λ = 1.59 Å) at the ILL neutron source, from samples contained within vanadium cans sealed under argon with indium washers. Rietveld profile refinements were performed using the GSAS suite of programs.15 Magnetization data were collected using a Quantum Design MPMS SQUID magnetometer. Thermogravimetric data were collected from powder samples under flowing oxygen using a Netzsch STA 409PC balance. X-ray absorption experiments were performed at beamline B18 of Diamond Light Source.16 The measurements were taken using the Ptcoated branch of collimating and focusing mirrors, a Si(111) doublecrystal monochromator, and a pair of harmonic rejection mirrors. The size of the beam at the sample position was approximately 600 μm (horizontal) × 700 μm (vertical). XANES data were collected at the Fe K-edge (7112 eV) and at the Ru K-edge (22123 eV) using Si (311) monochromator crystals and Pt-coated mirrors, oriented with a 2.3 mrad grazing incidence angle, for collimation and focusing. A pair of Pd-coated flat mirrors with a 9 mrad incidence angle were used to remove high-energy beam harmonic components for the Fe K-edge measurements. In both cases, data were collected in transmission mode with ion chambers before and behind the sample filled with appropriate mixtures of inert gases to optimize sensitivity [I0, 300 mbar of N2 and 700 mbar of He resulting in an overall efficiency of 10%; It, 150 mbar of Ar and 850 mbar of He, with 70% efficiency for Fe K-edge measurements; for Ru-edge measurements I0 of 250 mbar of Ar and 750 mbar of He and and It (Iref) of 350 mbar of Kr and 650 mbar of He]. The spectra were recorded with a step size equivalent to 0.25 eV (1 eV for the Ru K-edge) in continuous scan acquisition mode. Data were normalized using Athena17 with a linear pre-edge and polynomial postedge background subtracted from the raw ln(It/I0) data. The samples were prepared in the form of a self-supported pellet, with the thickness optimized to obtain an edge jump close to one. Room-temperature 57Fe Mössbauer spectra were recorded relative to α-Fe over the velocity range of ±5 mm/s using a constant acceleration spectrometer with a 25 mCi source of 57Co in Rh. Spectra were satisfactorily fit with three broadened Lorentzian paramagnetic doublets using the Recoil analysis software package.
strong reducing agents, such as binary metal hydrides, complex 4d and 5d transition metal oxides such as SrRuO3 decompose to the elemental transition metal, via nontopochemical processes, even at low temperatures.9 However, by incorporating first-row (3d) transition metals, the host lattice can be made sufficiently robust to resist decomposition upon reduction and allow the preparation of topochemically reduced phases. Thus, in contrast to the decomposition of SrRuO3 to SrO and Ru, SrFe 0.5 Ru 0.5 O 3 can be reduced with CaH 2 to form SrFe0.5Ru0.5O2, a phase in which both the iron and ruthenium cations adopt a square-planar local coordination and the first example of an extended oxide containing Ru2+ centers.9 Magnetic data and density functional theory calculations indicate that the iron centers adopt an S = 2, high-spin electronic configuration in common with the all-iron phase SrFeO2,10 while the ruthenium centers adopt S = 1, intermediate-spin configurations, as expected for a 4d transition metal.9 In this Article, we describe the reduction chemistry of the n = 1 and n = 2 Srn+1(Fe0.5Ru0.5)nO3n+1 Ruddlesden−Popper phases to extend the reduction chemistry of complex 4d transition metal oxides.
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EXPERIMENTAL SECTION
Synthesis of Srn+1(Fe0.5Ru0.5)nO3n+1 (n = 1 or 2). Samples of Srn+1(Fe0.5Ru0.5)nO3n+1 (n = 1 or 2) were prepared using a citrate gel method. Suitable quantities of SrCO3 (99.994%), RuO2 (99.99%, dried at 800 °C for 2 h), and Fe2O3 (99.99%) were dissolved in a minimal quantity of a 1:1 mixture of 6 M nitric acid and distilled water. Citric acid and analar ethylene glycol were added, and the solution was heated while being constantly stirred. The gels thus formed were subsequently ground into fine powders, placed in alumina crucibles, and heated in air at a rate of 1 °C/min to 1000 °C to remove the remaining organic components from the samples. The resulting powders were pressed into 13 mm diameter pellets and heated for two periods of 2 days at 1200 °C for the n = 1 sample and 1400 °C for the n = 2 sample, with regrinding between heating periods, as previously described by Battle et al.11 The materials produced were observed to be phase-pure by laboratory X-ray powder diffraction, with the following lattice parameters: a = 3.905(1) Å and c = 12.587(1) Å for Sr2Fe0.5Ru0.5O4, and a = 3.917(1) Å and c = 20.396(2) Å for Sr3(Fe0.5Ru0.5)2O7 (in good agreement with literature values).11 Reduction of Srn+1(Fe0.5Ru0.5)nO3n+1 (n = 1 or 2). The reduction of the Srn+1(Fe0.5Ru0.5)nO3n+1 (n = 1 or 2) phases was performed using CaH2 as a solid state reducing agent.4 To investigate the reactivity of the oxides with CaH 2 , small samples (∼200 mg) of Srn+1(Fe0.5Ru0.5)nO3n+1 (n = 1 or 2) phases were ground together with 2n molar equivalents of CaH2 in an argon-filled glovebox (
