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