Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

Journal name:
Nature Materials
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The discovery of high-temperature superconductivity in a layered iron arsenide1 has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers2, 3, 4, 5, 6, 7. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li+, Na+, K+, Ba2+) or PbO- or perovskite-type oxide layers, and also in Fe1.01Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Lix(NH2)y(NH3)1−yFe2Se2 (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.

At a glance


  1. The 298 K crystal structure of Li0.6(1)(ND2)0.2(1)(ND3)0.8(1)Fe2Se2.
    Figure 1: The 298 K crystal structure of Li0.6(1)(ND2)0.2(1)(ND3)0.8(1)Fe2Se2.

    Refinement was against neutron powder diffraction data (GEM instrument). In the model each square prism of Se atoms contains either an [ND2] anion or an ND3 molecule and these are both modelled as disordered over four orientations. The sizes of the spheres representing the Li atoms are in proportion to their site occupancies.

  2. Rietveld refinement against GEM data for Li0.6(1)(ND2)0.2(1)(ND3)0.8(1)Fe2Se2 at 298 K.
    Figure 2: Rietveld refinement against GEM data for Li0.6(1)(ND2)0.2(1)(ND3)0.8(1)Fe2Se2 at 298 K.

    The data are from the 2θ  =  64.6° detector bank. Allowed peak positions are marked by vertical lines: from the top, the main phase and minority phases: FeSe (hexagonal, present in the starting material; 8.8% by mass), FeSe (tetragonal; 1.4%), LiND2 (2.1%) and Fe (1.4%).

  3. Magnetic susceptibility measurements.
    Figure 3: Magnetic susceptibility measurements.

    ZFC and field-cooled measurements on the three samples of Li0.6(1)(NH/D2)0.2(1)(NH/D3)0.8(1)Fe2Se2 measured using μSR and NPD. Red symbols: μSR sample; blue symbols: H-containing NPD sample; green symbols: D-containing NPD sample.

  4. Analysis of μSR spectroscopy measurements.
    Figure 4: Analysis of μSR spectroscopy measurements.

    a, Field width Br.m.s. as a function of temperature. The fitted line comprises the three dashed contributions summed in quadrature. b, A Uemura plot of Tcagainst superfluid stiffness ρs  =  c2/λab2 showing that Li0.6(1)(NH2)0.2(1)(NH3)0.8(1)Fe2Se2 falls on the main scaling line. Data for Fe1+δSe are shown for different pressures (ref. 29) and the plot is adapted from ref. 10.


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Author information


  1. Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK

    • Matthew Burrard-Lucas,
    • David G. Free,
    • Stefan J. Sedlmaier,
    • Simon J. Cassidy,
    • Yoshiaki Hara,
    • Alex J. Corkett &
    • Simon J. Clarke
  2. Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK

    • Jack D. Wright &
    • Stephen J. Blundell
  3. Ibaraki National College of Technology, Department of Natural Science, Nakane 866, Hitachinaka 312-8508, Japan

    • Yoshiaki Hara
  4. Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

    • Tom Lancaster
  5. ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK

    • Peter J. Baker


M.B-L., D.G.F., Y.H. and S.J.S. prepared the samples, D.G.F., A.J.C., S.J.S. and S.J. Clarke performed the diffraction data collection and structural analysis. J.D.W., T.L., P.J.B. and S.J.B. performed the μSR measurements, M.B-L., S.J. Cassidy, A.J.C. and S.J. Clarke performed the magnetometry and other characterization measurements. S.J. Clarke conceived the project and, with S.J.B., wrote the manuscript.

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