Entropy stabilization and effect of A-site ionic size in bilayer nickelates

The discovery of high-temperature superconductivity in La$3$Ni$2$O$7$ under high pressure has sparked a surge of research into Ruddlesden-Popper (RP) nickelates. Currently, stabilizing the bilayer RP phases with smaller $A$-site ions remains a significant challenge. In this work, we have successfully synthesized medium- and high-entropy bilayer nickelates, La${1.2}$Pr${0.6}$Nd${0.6}$Sm${0.6}$Ni$2$O${7-δ}$ and La${0.67}$Pr${0.67}$Nd${0.67}$Sm${0.33}$Eu${0.33}$Gd$_{0.33}$Ni$2$O${7-δ}$, by utilizing the concept of configuration entropy stabilization. The high-entropy nickelate exhibits the smallest unit-cell volume and the largest orthorhombic distortion reported to date. The chemical pressure induced by the smaller A-site ions significantly enhances the NiO$_6$ octahedral rotation/distortion and shortens the interlayer Ni-Ni interatomic spacing. Physical property measurements reveal bad electrical conductivity alongside a markedly elevated density-wave transition temperature. Notably, the superconducting transition temperature extrapolated from structural correlations is projected to exceed 100 K. Our work not only demonstrates entropy stabilization of bilayer nickelates, but also reveals the effect of $A$-site-ion size on the crystal structure and physical properties, opening a new pathway for developing nickelate superconductors and tuning their electronic properties.

💡 Research Summary

The paper reports the successful synthesis of medium‑entropy (ME‑327) and high‑entropy (HE‑327) bilayer Ruddlesden‑Popper nickelates by deliberately mixing multiple rare‑earth cations on the A‑site. The compositions, La₁.₂Pr₀.₆Nd₀.₆Sm₀.₆Ni₂O₇₋δ (ME) and La₀.₆₇Pr₀.₆₇Nd₀.₆₇Sm₀.₃₃Eu₀.₃₃Gd₀.₃₃Ni₂O₇₋δ (HE), achieve configurational entropies of 1.31 R and 1.64 R respectively, satisfying the criteria for medium‑ and high‑entropy oxides. By selecting A‑site ions with an average ionic radius ⟨r_A⟩ of 1.181 Å (ME) and 1.164 Å (HE), the authors impose a substantial “chemical pressure” that mimics several gigapascals of external pressure while preserving the bilayer 327 structure.

X‑ray diffraction and Rietveld refinements confirm single‑phase materials with high crystallinity. Systematic peak shifts indicate contraction of the a and c lattice parameters and an expansion of the b parameter, leading to a pronounced orthorhombic distortion (ε up to ~1.9 % in the HE sample, the largest reported for any 327 nickelate). High‑resolution TEM and HAADF‑EDS mapping show homogeneous distribution of the rare‑earth elements, preferential occupation of the perovskite‑type A₂ layer, and a reduction of the inter‑layer Ni–Ni distance from 4.03 Å to 3.94 Å (≈2 % shortening). This structural change is expected to strengthen inter‑layer super‑exchange pathways that are thought to be crucial for superconductivity under high pressure.

Thermogravimetric analysis yields oxygen stoichiometries of O₆.₉₈ (ME) and O₆.₉₇ (HE), confirming near‑full oxygenation. Electrical resistivity measurements reveal semiconducting behavior for both entropy‑stabilized samples, with significantly higher resistivity than La‑327. Nevertheless, the derivative dρ/dT displays clear anomalies associated with a density‑wave (DW) transition at 159 K (ME) and 168 K (HE), markedly higher than the 144 K DW transition in pristine La‑327. The upward shift of the DW temperature parallels the effect of external pressure, indicating that chemical pressure effectively tunes the electronic instability.

Magnetic susceptibility follows an extended Curie–Weiss law, allowing separation of the rare‑earth local moments from the Ni contribution. After subtraction, a broad hump in the residual susceptibility appears around 150–200 K, consistent with the DW transition identified in transport. The effective magnetic moments derived from the Curie constant agree with the expected values for the mixed rare‑earth composition.

By correlating ⟨r_A⟩ with unit‑cell volume, orthorhombicity, and DW temperature, the authors demonstrate that decreasing ⟨r_A⟩ reproduces the volumetric effect of ≈4.3 GPa of physical pressure but produces a distinct structural response: enhanced orthorhombic distortion rather than a move toward tetragonal symmetry. The shortened Ni–Ni spacing and increased octahedral tilting are argued to narrow the Ni‑3d bandwidth, promoting carrier localization and raising the DW transition temperature.

Extrapolating from existing high‑pressure data on related 327 nickelates, the authors predict that the HE‑327 composition could achieve a superconducting transition temperature (Tc) exceeding 100 K under sufficient external pressure. Preliminary high‑pressure resistivity measurements at 31 GPa indeed show an anomaly near 103 K, suggestive of a superconducting onset.

In summary, the work establishes that high‑entropy design is an effective route to stabilize otherwise metastable bilayer nickelates, and that systematic reduction of the A‑site ionic radius provides a powerful “chemical pressure” lever to tune lattice distortions, density‑wave order, and ultimately the superconducting Tc. The study opens pathways for further exploration of single‑crystal and thin‑film HE nickelates, which will be essential for probing intrinsic anisotropic properties and for potential device applications.