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Hidden Charge Order in an Iron Oxide Square-Lattice Compound

Jung-Hwa Kim, Darren C. Peets, Manfred Reehuis, Peter Adler, Andrey Maljuk, Tobias Ritschel, Morgan C. Allison, Jochen Geck, Jose R. L. Mardegan, Pablo J. Bereciartua Perez, Sonia Francoual, Andrew C. Walters, Thomas Keller, Paula M. Abdala, Philip Pattison, Pinder Dosanjh, and Bernhard Keimer
Phys. Rev. Lett. 127, 097203 – Published 27 August 2021
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    Abstract

    Since the discovery of charge disproportionation in the FeO2 square-lattice compound Sr3Fe2O7 by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained “hidden” to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO2 planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.

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    • Received 7 April 2020
    • Accepted 4 August 2021

    DOI:https://doi.org/10.1103/PhysRevLett.127.097203

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Charge order
    1. Physical Systems
    Oxides
    1. Techniques
    Mössbauer spectroscopyNeutron diffractionX-ray diffraction
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Jung-Hwa Kim1,*, Darren C. Peets1,2,3,*, Manfred Reehuis4, Peter Adler5, Andrey Maljuk1,6, Tobias Ritschel3, Morgan C. Allison3, Jochen Geck3,7, Jose R. L. Mardegan8, Pablo J. Bereciartua Perez8, Sonia Francoual8, Andrew C. Walters1,9, Thomas Keller1,10, Paula M. Abdala11, Philip Pattison11,12, Pinder Dosanjh13, and Bernhard Keimer1,†

    • 1Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany
    • 2Ningbo Institute for Materials Technology and Engineering, Chinese Academy of Sciences, Zhenhai, Ningbo, 315201 Zhejiang, China
    • 3Institut für Festkörper- und Materialphysik, Technische Universität Dresden, D-01069 Dresden, Germany
    • 4Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany
    • 5Max-Planck-Institut für Chemische Physik fester Stoffe, D-01187 Dresden, Germany
    • 6Leibniz Institut für Festkörper- und Werkstoffforschung, D-01171 Dresden, Germany
    • 7Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden, 01062 Dresden, Germany
    • 8Deutsches Elektronen-Synchrotron DESY, Hamburg 22603, Germany
    • 9Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
    • 10Max Planck Society Outstation at the Heinz Maier-Leibnitz Zentrum (MLZ), D-85748 Garching, Germany
    • 11SNBL at ESRF, BP 220, F-38042 Grenoble Cedex 9, France
    • 12Laboratory for Quantum Magnetism, École polytechnique fédérale de Lausanne (EPFL), BSP-Dorigny, CH-1015 Lausanne, Switzerland
    • 13Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1 Canada

    • *These authors contributed equally to this work.
    • †b.keimer@fkf.mpg.de

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    Issue

    Vol. 127, Iss. 9 — 27 August 2021

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    • Figure 1
      Figure 1

      Schematic crystal structures of (a) charge-disordered metallic and (b) charge-ordered insulating Sr3Fe2O7; colors indicate the Fe valence states. (c) Specific heat. An entropy-conserving construction identifies a transition at TCO=332  K (dashed line), consistent with transport and diffraction data. (d) In-plane resistivity showing a metal-insulator transition at TCO. The anomalies at T∼115  K in (c) and (d) are due to the onset of helical magnetic order. (e) Mössbauer spectra of Sr3Fe2O7 in the paramagnetic and magnetically ordered phases. The outer and inner components correspond to Fe3+- and Fe5+-like sites, respectively [6, 7, 8, 9, 29]. Inset: two degenerate stacking patterns of Fe3+- and Fe5+-like sites in adjacent bilayers.

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    • Figure 2
      Figure 2

      High-resolution synchrotron x-ray powder diffraction pattern of the charge-ordered phase at T=15  K. Inset: tetragonal (2 1 9) and (2 2 0) Bragg peaks, together with the results of refinements in the I4/mmm and Bmmb space groups.

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    • Figure 3
      Figure 3

      Neutron Larmor diffraction results. (a) Splitting of the in-plane lattice parameters, Δd/d=(b−a)/a, extracted from the tetragonal (2 2 0) Bragg peak as a function of temperature T. In the analysis, the intrinsic peak width of ΔQ/Q=4.8×10−4 at 400 K was kept constant at all T. (b) Thermal expansion extracted from the cumulative Larmor phase of the tetragonal (2 2 0) and (0 0 10) Bragg peaks, relative to the phase at T=17  K.

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    • Figure 4
      Figure 4

      (a) Temperature dependence of the integrated intensity of the (12 12 14) superstructure reflection measured by Fe K-edge REXS slightly off resonance (photon energy 7112 eV). The line is a guide to the eye. Inset: in-plane K scans at temperatures marked by squares in the main panel, demonstrating the absence of T-dependent shifts or broadening of this reflection. (b) L scan on resonance at T=30  K. The line is the result of a calculation that considers only the Fe sites, assuming differing charge. The Fe positions were taken from the crystallographic refinement [29], and only the width, overall intensity, and the imbalance between the population of orthorhombic twin domains (45:55%) were fitted. Reciprocal-space locations refer to the high-temperature I4/mmm cell. (12 12 14) in I4/mmm is equivalent to (1 0 14)/(0 1 14) in Bmmb.

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