🔬 materials science

Bulk superconductivity in quintuple-layer square-planar nickelate La6Ni5O12

This paper reports the discovery of bulk superconductivity at ambient pressure in the quintuple-layer square-planar nickelate La6Ni5O12, synthesized via topotactic reduction, which exhibits an onset transition temperature of approximately 18 K and a superconducting volume fraction of ~25%, thereby establishing a crucial platform for investigating the mechanism of high-temperature superconductivity in nickelates.

Original authors: Mengzhu Shi, Mei Du, Hongyu Li, Zhiwei Wang, Zhengjie Wang, Ziji Xiang, Tao Wu, Xianhui Chen

Published 2026-10-15

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CC BY 4.0

Original authors: Mengzhu Shi, Mei Du, Hongyu Li, Zhiwei Wang, Zhengjie Wang, Ziji Xiang, Tao Wu, Xianhui Chen

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, the most famous family of materials that conducts electricity without any resistance at all has been copper-based ceramics, known as cuprates. These materials can carry current with perfect efficiency at temperatures far warmer than absolute zero, a property that has fascinated scientists since the 1980s. However, understanding exactly how they work has been difficult because the most useful versions of these materials only exist as thin films or require crushing pressures to function. This limitation has forced researchers to rely on indirect methods to study them, much like trying to understand the interior of a house by only looking at the roof. To solve this puzzle, scientists have been searching for a new family of materials that behaves like the copper ones but is easier to study. They turned their attention to nickel-based oxides, which share a similar chemical structure and electronic environment with the copper materials. While nickel-based superconductors were discovered recently, they too were stuck in the same trap: they only worked in thin layers or under extreme pressure, leaving the bulk of the material in a non-superconducting state.

A team of researchers has now broken this barrier by creating a chunk of nickel-based material that conducts electricity without resistance at normal atmospheric pressure. The scientists started with a specific nickel compound, a five-layered crystal structure that naturally forms under high heat and pressure. This starting material was a metallic conductor, but it was not yet a superconductor. To transform it, the researchers performed a delicate chemical surgery. They placed the crystals in a sealed environment with a chemical agent that acts like a sponge, soaking up specific oxygen atoms from the crystal's surface and interior. This process, known as a topotactic reaction, removed just enough oxygen to change the arrangement of the atoms inside the material without breaking the crystal apart. The result was a new version of the material where the nickel atoms sit in flat, square-like planes, a structure that theoretical models had long predicted would be ideal for superconductivity.

When the researchers tested this new material, they found that it began to conduct electricity with zero resistance at a temperature of about 18 Kelvin, which is roughly 255 degrees below zero on the Celsius scale. This was not just a surface effect; the material behaved as a true superconductor throughout its entire volume. To confirm this, the team measured how the material responded to magnetic fields. They observed that the material actively pushed magnetic fields away from itself, a hallmark of superconductivity, and that it could trap magnetic lines of force within its structure, a sign that it was carrying electrical currents without losing energy. They also measured the heat capacity of the material, which showed a distinct jump at the same temperature where the electricity stopped meeting resistance. This jump is a definitive thermodynamic signature that a large portion of the material had entered a superconducting state.

The study revealed that the success of this transformation depended entirely on precision. If the chemical treatment was too weak, the material remained a normal metal with some unusual magnetic properties. If the treatment was too strong, the material turned into an insulator that blocked electricity entirely. Only within a very narrow window of chemical adjustment did the superconducting state appear. The researchers estimated that about one-quarter of the material in their sample was superconducting, with the rest remaining in a normal state, likely due to tiny cracks or slight variations in the chemical makeup. Despite this, the discovery is significant because it provides the first bulk sample of a nickel-based superconductor that works at normal pressure. This opens the door for scientists to use powerful tools like neutron scattering and advanced imaging to look directly inside the material, potentially revealing the secrets of high-temperature superconductivity that have remained hidden for years.

Technical Summary: Bulk Superconductivity in Quintuple-Layer Square-Planar Nickelate La₆Ni₅O₁₂

Problem Statement
Since the discovery of cuprate superconductors, nickel-based oxides have been investigated as structural and electronic analogs to unravel the mechanism of high-temperature superconductivity. While superconductivity has been confirmed in infinite-layer nickelate thin films (e.g., Nd₁₋ₓSrₓNiO₂) and various multilayer nickelates (including square-planar and Ruddlesden–Popper phases), all such discoveries to date have been restricted to thin-film architectures or samples under high pressure. This limitation prevents the application of bulk-sensitive experimental probes (such as neutron scattering and nuclear magnetic resonance) and hinders the study of intrinsic bulk electronic properties without interfacial or strain ambiguities. Previous attempts to synthesize bulk single crystals of infinite-layer nickelates via topotactic reduction have failed to yield superconductivity, often resulting in insulating behavior due to structural disorder, microcracks, and disrupted intragrain continuity. Consequently, the realization of bulk superconductivity in nickelates at ambient pressure remains an unachieved goal.

Methodology
The authors employed a two-step synthesis strategy to overcome these limitations:

High-Pressure/High-Temperature Synthesis: They first synthesized the quintuple-layer Ruddlesden–Popper (RP) precursor, La₆Ni₅O₁₆, in bulk single-crystal form using a six-anvil high-pressure apparatus (5 GPa, 1600 °C). Single-crystal X-ray diffraction and scanning transmission electron microscopy (STEM) confirmed the monoclinic RP structure comprising five perovskite (LaNiO₃)₅ sublayers separated by rock-salt LaO layers.
Topotactic Reduction: The precursor crystals underwent topotactic reduction using LiAlH₄ as an oxygen getter in a sealed quartz tube. The reaction was conducted at varying temperatures (310 °C, 320 °C, and 330 °C) for 40 hours to remove apical oxygen atoms from the NiO₆ octahedra. This process aimed to convert the perovskite-derived layers into square-planar (SP) NiO₂ planes, analogous to cuprates, forming the target phase La₆Ni₅O₁₂.
Characterization: The resulting samples were characterized using powder X-ray diffraction (XRD), HAADF-STEM, and iDPC-STEM to verify structural transformation. Transport properties were measured via four-probe resistivity on selected microcrystals. Magnetic properties were probed using zero-field-cooled (ZFC)/field-cooled (FC) susceptibility, magnetization hysteresis loops, and highly sensitive magnetic torque measurements on individual crystals. Specific heat measurements were performed on polycrystalline pellets to assess thermodynamic bulk properties.

Key Results

Structural Transformation: Topotactic reduction successfully converted monoclinic La₆Ni₅O₁₆ into tetragonal T′-type La₆Ni₅O₁₂. STEM imaging confirmed the removal of apical oxygen and the formation of five alternating infinite-layer NiO₂ planes and fluorite-type blocking layers.
Optimization Window: The electronic state of La₆Ni₅O₁₂ is highly sensitive to the reduction temperature.
- At 310 °C, samples exhibited a metallic state with a density-wave-like anomaly near 94 K and a superconducting transition onset near 18 K, though with residual resistance.
- At 320 °C, the samples displayed a clean metallic normal state followed by a sharp superconducting transition with an onset temperature ( $T_c$ ) of approximately 18 K and zero resistance.
- At 330 °C, samples became insulating, indicating over-reduction.
Superconducting Evidence:
- Transport: Resistivity measurements showed a transition to zero resistance at ~18 K for the 320 °C-annealed sample. The transition was suppressed by magnetic fields, yielding an upper critical field $\mu_0H_{c2}(0) \approx 41.5$ T.
- Magnetism: ZFC/FC susceptibility measurements revealed a diamagnetic shielding fraction of ~26% at 18 K. Magnetization hysteresis loops at 2 K demonstrated type-II superconductivity with a lower critical field $\mu_0H_{c1} \approx 950$ Oe and a critical current density $J_c \approx 2000$ A cm⁻². Magnetic torque measurements on single crystals confirmed bulk superconductivity and flux pinning.
- Specific Heat: Specific heat measurements on polycrystalline pellets showed a clear jump anomaly at ~18 K, shifting to lower temperatures under magnetic fields. Analysis of the electronic specific heat ( $C_e$ ) indicated a superconducting volume fraction of approximately 25%, consistent with the magnetic shielding data. The data fit a single-band BCS $\alpha$ -model with a mean-field $T_c$ of 17.5 K and a gap $\Delta = 3.6$ meV.

Significance and Claims
The paper claims the first realization of bulk superconductivity at ambient pressure in a nickelate superconductor, specifically the quintuple-layer square-planar compound La₆Ni₅O₁₂. The authors emphasize that this discovery establishes an unprecedented platform for investigating nickelate superconductivity using bulk-sensitive techniques that were previously inaccessible due to the thin-film nature of prior samples.

The work supports theoretical predictions that the $n=5$ member of the square-planar nickelate family naturally resides in a doping regime favorable for superconductivity, with a nominal Ni valence of +1.2 comparable to optimally doped infinite-layer thin films. The authors note that the availability of these bulk crystals removes interfacial and strain-related ambiguities, paving the way for future experimental studies into the superconducting mechanism, particularly regarding the role of multi-layer electronic correlations and comparisons with infinite-layer thin films. The paper concludes that this platform enables systematic optimization of superconducting properties through chemical substitution and structural engineering, encouraging the search for additional ambient-pressure nickelate superconductors within the broader Ruddlesden–Popper series.

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