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

The observation of high-Tc superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in the bilayer La₃Ni₂O₇ has sparked hopes of achieving HTSC by stabilizing the tetragonal phase at ambient pressure. To mimic the effect of external physical pressures, the application of chemical pressure via replacing La³^+$ with smaller rare-earth R$^3⁺ has been considered as a potential route. Here we clarify the distinct effects of chemical and physical pressures on the structural transition of bilayer nickelates through a combined experimental and theoretical investigation. Contrary to general expectations, we find that substitutions of smaller R³^+$ for La$^3⁺ in La₃₋ₓRₓNi₂O7-δ, despite of an overall lattice contraction, produce stronger orthorhombic structural distortions and thus require higher pressures to induce the structural transition. We established a quantitative relationship between the critical pressure Pc for structural transition and the average size of A-site cations. A linear extrapolation of Pc versus <rA> yields a putative critical value of <rA>c ~ 1.23 angstrom for Pc ~ 1 bar. The negative correlation between Pc and <rA> indicates that it is unlikely to reduce Pc to ambient by replacing La³^+$ with smaller R$^3⁺ ions. Instead, partial substitution of La³^+$ with larger cations such as alkaline-earth Sr$^2⁺ or Ba²^+$ might be a feasible approach. Our results provide valuable guidelines in the quest of ambient-pressure HTSC in bilayer nickelates.