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

TL;DR

This study demonstrates entropy stabilization of bilayer nickelates with smaller A-site ions, revealing structural and electronic effects that could lead to high-temperature superconductivity above 100 K.

Contribution

The paper introduces the synthesis of high-entropy bilayer nickelates using entropy stabilization, showing their unique structural properties and potential for high-temperature superconductivity.

Findings

Smallest unit-cell volume reported for nickelates

Enhanced octahedral distortion due to chemical pressure

Projected superconducting T_c exceeds 100 K

Abstract

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$_{7-\delta}$ and La$_{0.67}$Pr$_{0.67}$Nd$_{0.67}$Sm$_{0.33}$Eu$_{0.33}$Gd$_{0.33}$Ni$_2$O$_{7-\delta}$, 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.