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Formation of single-phase disordered CsxFe2−ySe2 at high pressure

V. Svitlyk, E. Pomjakushina, A. Krzton-Maziopa, K. Conder, and M. Mezouar
Phys. Rev. B 97, 214512 – Published 15 June 2018
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  • INTRODUCTION
  • EXPERIMENT
  • RESULTS AND DISCUSSION
  • CONCLUSIONS
    • Supplemental Material
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    Abstract

    A single-phase high-pressure (HP) modification of CsxFe2−ySe2 was synthesized at 11.8 GPa at ambient temperature. Structurally this polymorph is similar to the minor low-pressure (LP) superconducting phase; namely, they both crystallize in a ThCr2Si2-type structure without ordering of the Fe vacancies within the Fe-deficient FeSe4 layers. The HP CsxFe2−ySe2 polymorph is found to be less crystalline and nearly twice as soft compared to the parent major and minor phases of CsxFe2−ySe2. It can be quenched to low pressures and is stable at least on the scale of weeks. At ambient pressure the HP polymorph of CsxFe2−ySe2 is expected to exhibit different superconducting properties compared to its LP minor phase (Tc=27K).

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    • Received 6 December 2017
    • Revised 10 April 2018

    DOI:https://doi.org/10.1103/PhysRevB.97.214512

    ©2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Crystal structurePressure effectsSuperconductivity
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    V. Svitlyk1,*, E. Pomjakushina2, A. Krzton-Maziopa3, K. Conder2, and M. Mezouar1

    • 1ID27 High Pressure Beamline, European Synchrotron Radiation Facility, 38000 Grenoble, France
    • 2Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, 5232 Villigen, Switzerland
    • 3Warsaw University of Technology, Faculty of Chemistry, 00-664 Warsaw, Poland

    • *svitlyk@esrf.fr

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    Issue

    Vol. 97, Iss. 21 — 1 June 2018

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    Images

    • Figure 1
      Figure 1

      Reconstruction of the hk0 reciprocal layers of CsxFe2−ySe2 at 0.2 (left) and 13.2 GPa (right). Left: The yellow grid corresponds to a reciprocal lattice of the average I4/mmm structure of CsxFe2−ySe2; a star indicates a group of 5×5 superstructure reflections (solid and dashed lines correspond to two different twin domains); a separate arrow indicates a slice through a diffuse rod of the minor phase [25]. Right: Yellow arrows mark a new lattice of the HP CsxFe2−ySe2 structure (I4/mmm symmetry). Additional reflections visible at 13.2 GPa (right) are not commensurate either with the main or the minor phases of CsxFe2−ySe2 and originate from the sample environment (diamonds, solid He/ruby crystals).

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    • Figure 2
      Figure 2

      Evolution of fine structural features of CsxFe2−ySe2 as a function of pressure. Top: High-resolution zoom on a region of the reciprocal space containing 020 reflections of the main and minor phases; positions of the reflections of the main phase were kept fixed as a reference. Middle: Corresponding profiles of the 020 reflections of the main and minor phases. Bottom: 5×5 superstructure reflections of the main phase and a slice through the diffuse rod of the minor phase.

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    • Figure 3
      Figure 3

      Evolution of fine structural parameters of CsxFe2−ySe2 as a function of pressure. Top left: Intensity ratio of the minor and main phases; insets show the 020 reflections. Top right: Intensity of 5×5 superstructure reflections. Bottom left: Experimental a parameters as directly refined from the behavior of the 020 reflection of the main and secondary phases (Fig. 2). Bottom right: c parameter (extracted from the pair of the 033¯ and 020 reflections) and unit-cell volume of the main phase; insets show the 033¯ reflections.

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    • Figure 4
      Figure 4

      Average model of the HP CsxFe2−ySe2 phase corresponding to the ThCr2Si2-type arrangement (I4/mmm). Fe in the layers of the edge-shared FeSe4 tetrahedra (orange) are 3/4 occupied; intercalated Cs atoms (dark gray) also occupy about 3/4 of available positions (Cs vacancies are highlighted).

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    • Figure 5
      Figure 5

      Left: Panoramic projection of the CsxFe2−ySe2 single crystal kept at 0.7 GPa for ten days. Right: Profile of the 020 reflection highlighting the single-phase nature of the sample. Similar to Fig. 1, additional reflections visible in data taken at 0.7 GPa (left) are not commensurate with either the main (including 5×5 superstructure reflections) or the minor phases of CsxFe2−ySe2 and originate from the sample environment.

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    • Figure 6
      Figure 6

      Evolution of CsxFe2−ySe2 with pressure at 20 K.

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