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

Abstract

The observation of high-$T_c$ superconductivity (HTSC) in concomitant with pressure-induced orthorhombic-tetragonal structural transition in the bilayer La${}_3$Ni${}_2$O${}_7$ 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 La${}^3$${}^{+}$ with smaller rare-earth R${}^3$${}^{+}$ 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 R${}^3$${}^{+}$ for La${}^3$${}^{+}$ in La${}_{3-x}$R${}_x$Ni${}_2$O${}_{7-\hat{I}´}$, 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 $P_c$ for structural transition and the average size of $A$-site cations. A linear extrapolation of $P_c$ versus <$r_A$> yields a putative critical value of <$r_A$>${}_c$ ~ 1.23 angstrom for $P_c$ ~ 1 bar. The negative correlation between $P_c$ and <$r_A$> indicates that it is unlikely to reduce $P_c$ to ambient by replacing La${}^3$${}^{+}$ with smaller R${}^3$${}^{+}$ ions. Instead, partial substitution of La${}^3$${}^{+}$ with larger cations such as alkaline-earth Sr${}^2$${}^{+}$ or Ba${}^2$${}^{+}$ might be a feasible approach. Our results provide valuable guidelines in the quest of ambient-pressure HTSC in bilayer nickelates.

17 pages and 5 figures