Source capture, zotero
Authors Ryo Misawa, Shunsuke Kitou, Jian-Ping Sun, Yingpeng Yu, Chihaya Koyama, Yuiga Nakamura, Taka-hisa Arima, Jin-Guang Cheng, Max Hirschberger
arXiv / Publisher https://arxiv.org/abs/2603.25119
PDF https://arxiv.org/pdf/2603.25119
Relevance score 5.718
Primary category cond-mat.supr-con
Published 2026-03-27
Research paradigm Experimental
Sample form Single Crystal

Summary

This study employs high-brightness synchrotron X-ray diffraction for precise structural analysis of high-quality single crystals of the bilayer nickelate La3Ni2O7. Using a large dynamic range detector, we successfully resolved previously overlooked weak diffraction signals, whose intensities are nearly four orders of magnitude weaker than the main Bragg reflections. These observations indicate the presence of glide mirror symmetry breaking in the crystal, leading to a polar structure (space group Ima2) instead of the previously assumed centrosymmetric model (Fmmm). Further structural refinement reveals two inequivalent nickel sites with significantly different Ni–O bond lengths. Combined with bond valence sum calculations, this suggests a checkerboard-like charge ordering of nickel sites, which, together with oxygen octahedral tilting, endows the crystal with polarity. The charge-ordered phase is structurally analogous to the polar state observed in bilayer manganites. This study establishes the polar charge-ordered state of La3Ni2O7 at ambient pressure, indicating its competition with pressure-induced superconductivity, and provides critical structural insights for understanding phase competition mechanisms and the origin of pressure-induced superconductivity in bilayer nickelates.

Materials

Methods

Keywords

Highlights

  • Resolution of faint reflections nearly four orders of magnitude weaker than main Bragg reflections that were overlooked in prior diffraction studies.

Conclusions

  • Evidence of a broken glide-mirror symmetry leading to a polar crystal structure, rather than the widely used centrosymmetric structure model.
  • The polarity is induced by checkerboard charge order on nickel sites in combination with octahedral tilting.

Main claims

  • Synchrotron XRD reveals broken glide-mirror symmetry, leading to a polar crystal structure (Ima2) rather than the widely used centrosymmetric Fmmm.
    • Evidence: Abstract,Full text: This observation evidences a broken glide-mirror symmetry, leading to a polar crystal structure, rather than the widely used centrosymmetric structure model.
  • The polarity is induced by checkerboard charge order on nickel sites in combination with octahedral tilting.
    • Evidence: Abstract,Full text: The polarity is induced by checkerboard charge order on nickel sites in combination with octahedral tilting.
  • The refined structure contains two inequivalent Ni sites with bond-length difference ≈0.25 Å, corresponding to a valence difference of ≈0.5 e.
    • Evidence: Full text,Section: The nickel site with a contracted octahedron has a valence of +2.6, whereas the other nickel site has a valence of +2.1.

Workflow

  • crystal_growth
    • Materials: La3Ni2O7
    • Methods: single crystal growth
    • Observations: high-quality single crystal
  • XRD_measurement
    • Methods: synchrotron X-ray diffraction; area detector with large dynamic range
    • Observations: faint reflections breaking glide-mirror symmetry; systematic presence of (h0l) with h odd
  • structure_refinement
    • Methods: crystallographic refinement in Ima2 and Fmmm
    • Observations: Ima2 gives much better fit (R=2.07%); two inequivalent Ni sites with bond-length difference 0.25 Å
  • charge_order_analysis
    • Methods: bond-valence sum calculation
    • Observations: Ni valence difference ≈0.5 e; checkerboard charge order
  • interpretation — Polar structure arises from checkerboard charge order combined with octahedral tilting, analogous to bilayer manganites.