Chemical versus physical pressure effects on the structure transition of bilayer nickelates

The observation of high-T_c superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in bilayer La₃Ni₂O₇ has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. Chemical pressure, introduced by replacing La³⁺ with smaller rare-earth R³⁺ has been considered as a potential route. However, our experimental and theoretical investigation reveals that such substitutions, despite causing lattice contraction, actually produce stronger orthorhombic distortions, requiring higher pressures for the structural transition. A linear extrapolation of P_c versus the average size of A-site cations (<r_A>), yields a putative critical value of <r_A>_c ≈ 1.23 Å for P_c ≈ 1 bar. The negative correlation between P_c and <r_A> indicates that replacing La³⁺ with smaller R³⁺ ions is unlikely to reduce P_c to ambient pressure. Instead, substituting La³⁺ with larger cations like Sr²⁺ or Ba²⁺ might be a feasible approach. Our results provide guidance for realizing ambient-pressure HTSC in bilayer nickelates.