Key Takeaways & Executive Findings
- •• • HE-327 (La0.67Pr0.67Nd0.67Sm0.33Eu0.33Gd0.33Ni2O7−δ) achieves a reduced average A-site ionic radius of 1.164 Å, leading to the lowest cell volume and largest orthorhombicity among bilayer nickelates, which is critical for enhancing interlayer coupling and superconductivity. • • Under 31 GPa, HE-327 exhibits a resistivity anomaly at 103 K, indicating a possible superconducting transition; extrapolated Tc exceeds 100 K, demonstrating that chemical pressure via ionic size reduction can significantly raise Tc. • • The density-wave transition temperature (TDW) increases linearly with decreasing rA, and HE-327 shows the highest TDW, suggesting a positive correlation between ambient-pressure DW order and high-pressure superconductivity. • • The high-entropy strategy successfully stabilizes the 327 phase with small rA, yielding phase-pure and homogeneous samples, overcoming the inherent instability of La-327 and expanding the compositional space for bilayer nickelates.
Abstract
The discovery of high-temperature superconductivity in bilayer nickelate La3Ni2O7−δ (La-327) under high pressure and in thin films at ambient pressure has opened new avenues in superconductivity research. However, La-327 exhibits a narrow phase stability range, leading to stacking faults that suppress bulk superconductivity. Chemical substitutions, particularly at the A-site with smaller rare-earth ions, have been shown to enhance phase purity and reduce stacking faults, while also increasing the orthorhombic distortion and chemical pressure. In this work, we apply the high-entropy (HE) strategy to stabilize the 327 phase with reduced average A-site ionic radius (rA). We successfully synthesized medium-entropy La1.2Pr0.6Nd0.6Sm0.6Ni2O7−δ (ME-327) and high-entropy La0.67Pr0.67Nd0.67Sm0.33Eu0.33Gd0.33Ni2O7−δ (HE-327) polycrystalline samples. These compositions satisfy medium- and high-entropy criteria, with rA values of 1.181 Å and 1.164 Å, respectively. The samples are phase-pure and homogeneous. HE-327 exhibits the lowest cell volume, largest orthorhombicity, and shortest interlayer Ni-Ni distance among reported bilayer nickelates. Physical property measurements reveal low electrical conductivity and a high density-wave (DW) transition temperature. Under high pressure, HE-327 shows a resistivity anomaly at 103 K under 31 GPa, suggesting a possible superconducting transition. Extrapolation indicates that Tc under high pressure exceeds 100 K for HE-327, correlating with reduced rA and enhanced interlayer coupling. Our results demonstrate the ionic size effect and the effectiveness of the HE approach in stabilizing bilayer nickelates, providing a new avenue for developing superconducting materials.
1. Introduction
Bilayer nickelate superconductors, exemplified by La3Ni2O7−δ (La-327), have attracted intense interest following the discovery of high-temperature superconductivity under high pressure and in thin films. However, the narrow phase stability range of La-327 leads to stacking faults that suppress bulk superconductivity, limiting practical applications. Chemical substitutions at the A-site with smaller rare-earth ions have been shown to improve phase purity and reduce stacking faults, but the phase stability remains a bottleneck. The high-entropy (HE) strategy, which involves incorporating multiple elements into a specific crystallographic site, offers a promising route to stabilize phases that are otherwise difficult to synthesize. By maximizing configurational entropy, HE materials can achieve enhanced structural stability and tunable properties. Applying this approach to bilayer nickelates could expand their compositional space and potentially optimize pressurized superconductivity.
In this work, we report the successful synthesis of medium-entropy (ME-327) and high-entropy (HE-327) bilayer nickelates with significantly reduced average A-site ionic radius (rA). These compositions satisfy the entropy criteria and exhibit phase purity and homogeneity. The reduced rA induces chemical pressure, leading to lattice contraction and enhanced orthorhombic distortion. Our measurements reveal that HE-327 possesses the lowest cell volume, largest orthorhombicity, and shortest interlayer Ni-Ni distance among reported bilayer nickelates. Under high pressure, HE-327 shows a resistivity anomaly at 103 K under 31 GPa, suggesting a possible superconducting transition. Extrapolation indicates that Tc exceeds 100 K, correlating with reduced rA and enhanced interlayer coupling. These findings demonstrate the ionic size effect and the effectiveness of the HE approach in stabilizing bilayer nickelates, providing a new avenue for developing superconducting materials.
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Jia-Yi Lu, Jia-Xin Li, Xin-Yu Zhao, Ya-Nan Zhang, Yi-Qiang Lin, Kai-Xin Ye, Hui-Qiu Yuan, Guang-Han Cao (2026). Entropy stabilization and effect of A-site ionic size in bilayer nickelates. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3976-x
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Frequently Asked Questions
What is the maximum extrapolated superconducting transition temperature (Tc) for HE-327 under high pressure, and how does it compare to other bilayer nickelates?
The extrapolated Tc for HE-327 exceeds 100 K, which is higher than that of La-327 (around 80 K) and other reported bilayer nickelates. This enhancement is attributed to the reduced A-site ionic radius (rA = 1.164 Å) that increases chemical pressure and interlayer coupling.
How does the high-entropy strategy contribute to the phase stability of bilayer nickelates, and what are the specific entropy values for ME-327 and HE-327?
The high-entropy strategy stabilizes the 327 phase by maximizing configurational entropy. ME-327 has a configurational entropy between 1.0R and 1.5R (medium-entropy), while HE-327 has entropy ≥1.5R (high-entropy). This entropy stabilization allows the incorporation of smaller A-site cations, which would otherwise destabilize the phase.
What is the relationship between the average A-site ionic radius (rA) and the density-wave transition temperature (TDW) and superconducting transition temperature (Tc)?
Both TDW and Tc increase almost linearly with decreasing rA. HE-327, with the smallest rA (1.164 Å), exhibits the highest TDW and an extrapolated Tc exceeding 100 K. This correlation suggests that chemical pressure from smaller ions enhances interlayer coupling, which is beneficial for superconductivity.
What are the key structural parameters of HE-327 that distinguish it from other bilayer nickelates?
HE-327 possesses the lowest cell volume, largest orthorhombicity, and shortest interlayer Ni-Ni distance among reported bilayer nickelates. These structural features are a direct consequence of the reduced rA and contribute to enhanced interlayer coupling and superconductivity.
What are the potential challenges in scaling up the synthesis of HE-327 for practical applications?
The synthesis of HE-327 requires precise control of multiple rare-earth elements and high-pressure conditions for superconductivity. Scaling up may face challenges in maintaining phase purity and homogeneity, as well as the high cost of rare-earth elements. However, the high-entropy approach could be extended to thin-film growth, which may enable ambient-pressure superconductivity and device integration.
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