Chemical versus physical pressure effects on the structure transition of bilayer nickelates
Abstract
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 R3+ 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 La3+ 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 R3+ ions. Instead, partial substitution of La3+ with larger cations such as alkaline-earth Sr2+ or Ba2+ might be a feasible approach. Our results provide valuable guidelines in the quest of ambient-pressure HTSC in bilayer nickelates.