Research ArticleCHEMICAL PHYSICS

Verwey-type charge ordering transition in an open-shell p-electron compound

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Science Advances  19 Jan 2018:
Vol. 4, no. 1, eaap7581
DOI: 10.1126/sciadv.aap7581
  • Fig. 1 Structural properties of Cs4O6.

    (A) Powder neutron diffraction patterns at 2 and 375 K. Top: At 375 K, Cs4O6 crystallizes in the C structure with space group Embedded Image. Bottom: At 2 K, Cs4O6 crystallizes in the T structure with space group Embedded Image. Black points and red and blue lines correspond to experimental data, calculated pattern, and difference curve, respectively. Broad humps in the patterns are due to the quartz tube containing the air-sensitive sample. (B) Illustration of the C (top) and T (bottom) crystal structures of Cs4O6. In the C structure, the O2 units (brown) are indistinguishable, whereas the T structure features CO of distinct peroxide O22− (red) and superoxide O2 (blue) anions. Cs+ ions are shown as green spheres.

  • Fig. 2 Signatures of the Verwey-type transition in Cs4O6.

    (A) Temperature dependence of the lattice parameters acub of the C phase and atet and ctet of the T phase. On cooling, the C to T transformation starts at TS1 = 264 K and is completed at TS2 = 213 K. On heating, transformation back into the C phase occurs near TS3 = 315 K. (B) The structural transition is reflected in anomalies and a hysteresis in the magnetic susceptibilities χ(T), as apparent in the representation of χT versus T. The inset shows the low-temperature magnetic susceptibilities verifying the absence of long-range magnetic order down to 1.8 K. (C) Arrhenius plot of the conductivity of Cs4O6, featuring a hysteresis in the temperature range between 313 and 233 K. Blue and red lines are line fits for the determination of the activation energies EA1 to EA3.

  • Fig. 3 Raman spectrum of Cs4O6 powder at room temperature.

    The vibrations at 767 and 1139 cm−1 correspond to the stretching vibrations of the peroxide and superoxide anions, respectively.

  • Fig. 4 Complementary spectroscopic techniques probe charge dynamics in the C and T phases of Cs4O6.

    (A) Sketch of characteristic time scales covered by the indicated experimental techniques. Orange and blue shading indicates the time scale range, where charge delocalization or localization is observed by respective spectroscopic probes. (B) Because of rapid charge exchange compared to τNMR, no 17O NMR signal is observed in the C phase (top), whereas a quadrupole broadened powder spectrum due to nonmagnetic O22− units is measured in the T phase (bottom). (C) Similarly, rapid charge exchange compared to τEPR dramatically broadens the X-band EPR spectrum in the C phase (top), whereas charge localization leads to a broad EPR signal in the T phase (bottom), corresponding to EPR active O2. (D) Schematic picture of charge delocalization in the C phase (top), where the electronic wave function of the fourth electron (orange shaded area) extends over all three O2 dumbbells in the formula unit. In the T phase (bottom), the extra electron localizes and its wave function (blue shaded area) shrinks to a single O22− dumbbell, leaving the other two in the O2 paramagnetic state.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaap7581/DC1

    section S1. Temperature dependence of the X-band EPR spectra

    section S2. 17O NMR line shape modeling

    section S3. Temperature dependence of the 17O NMR spectra

    fig. S1. X-band EPR spectra of Cs4O6 for selected temperatures.

    fig. S2. Structural phase transition in Cs4O6.

    fig. S3. Time-dependent conductivity of a Cs4O6 pellet at 273 K.

    fig. S4. Temperature dependence of the charge carrier mobility for Cs4O6.

    fig. S5. Temperature dependence of the powder central transition 17O NMR spectra.

    fig. S6. Line shape modeling of powder 17O NMR spectrum of isotopically enriched Cs4O6.

    table S1. Results of the crystal structure refinements of Cs4O6 from PND data.

    table S2. Interatomic distances (in Å) as obtained from the crystal structure refinements of Cs4O6 PND data at 2 K.

    table S3. Results from DFT calculations.

  • Supplementary Materials

    This PDF file includes:

    • section S1. Temperature dependence of the X-band EPR spectra
    • section S2. 17O NMR line shape modeling
    • section S3. Temperature dependence of the 17O NMR spectra
    • fig. S1. X-band EPR spectra of Cs4O6 for selected temperatures.
    • fig. S2. Structural phase transition in Cs4O6.
    • fig. S3. Time-dependent conductivity of a Cs4O6 pellet at 273 K.
    • fig. S4. Temperature dependence of the charge carrier mobility for Cs4O6.
    • fig. S5. Temperature dependence of the powder central transition 17O NMR spectra.
    • fig. S6. Line shape modeling of powder 17O NMR spectrum of isotopically enriched Cs4O6.
    • table S1. Results of the crystal structure refinements of Cs4O6 from PND data.
    • table S2. Interatomic distances (in Å) as obtained from the crystal structure refinements of Cs4O6 PND data at 2 K.
    • table S3. Results from DFT calculations.

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