Polar, checkerboard charge order in bilayer nickelate La₃Ni₂O₇

摘要

本研究利用高亮度同步辐射X射线衍射对双层镍酸盐La₃Ni₂O₇的高质量单晶进行精确结构分析，通过大动态范围探测器成功解析了之前被忽略的弱衍射信号，这些信号的强度比主Bragg反射弱近四个数量级。这些观测证据表明，晶体中存在滑移镜面对称性破缺，导致其呈现极性结构（空间群Ima2），而非先前普遍采用的中心对称模型（Fmmm）。进一步的结构精修揭示了两个不等价镍位点，其Ni–O键长差异显著，结合键价和计算，表明镍位点电荷呈现棋盘状有序分布，并与氧八面体倾斜协同作用赋予晶体极性。该电荷有序相的结构与双层锰氧化物中的极化态类似。本研究确立了常压下La₃Ni₂O₇的极化电荷有序态，指出其与压力诱导的超导电性存在竞争关系，为理解双层镍酸盐中相竞争机制和压力超导起源提供了关键结构基础。

材料

• La₃Ni₂O₇

方法

• synchrotron X-ray diffraction

关键词

• charge order
• polar structure
• checkerboard order
• octahedral tilting
• phase competition

亮点

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

结论

• 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.

主要论断

• Synchrotron XRD reveals broken glide-mirror symmetry, leading to a polar crystal structure (Ima2) rather than the widely used centrosymmetric Fmmm.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.

研究流程

• crystal_growth
  • 材料: La₃Ni₂O₇
  • 方法: single crystal growth
  • 观察: high-quality single crystal
• XRD_measurement
  • 方法: synchrotron X-ray diffraction; area detector with large dynamic range
  • 观察: faint reflections breaking glide-mirror symmetry; systematic presence of (h0l) with h odd
• structure_refinement
  • 方法: crystallographic refinement in Ima2 and Fmmm
  • 观察: Ima2 gives much better fit (R=2.07%); two inequivalent Ni sites with bond-length difference 0.25 Å
• charge_order_analysis
  • 方法: bond-valence sum calculation
  • 观察: 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.