• • 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.