Geometric effects in the infinite-layer nickelates

TL;DR

This study investigates how the size of the $R$-site atom affects the structure and electronic properties of infinite-layer nickelates, revealing a geometric-induced quantum critical point and structural instability that can be tuned by strain or atom size.

Contribution

It demonstrates the impact of $R$-site atom size on the structural and electronic properties of $R$NiO$_2$, identifying a tunable quantum critical point and structural instability.

Findings

La to Y substitution is equivalent to 19 GPa pressure.

Presence of topotactic hydrogen can be excluded.

Geometric effects induce a quantum critical point and structural transition.

Abstract

Geometric effects in the infinite-layer nickelates $R$NiO$_2$ associated with the relative size of the $R$-site atom are investigated via first-principles calculations. We consider, in particular, the prospective YNiO$_2$ material to illustrate the impact of these effects. Compared to LaNiO$_2$, we find that the La $\to$ Y substitution is equivalent to a pressure of 19 GPa and that the presence of topotactic hydrogen can be precluded. However, the electronic structure of YNiO$_2$ departs from the cuprate-like picture due to an increase in both self-doping effect and $e_g$ hybridization. Furthermore, we find that geometric effects introduce a quantum critical point in the $R$NiO$_2$ series. This implies a $P4/mmm \leftrightarrow I4/mcm$ structural transformation associated to a $A_3^+$ normal mode, according to which the oxygen squares undergo an in-plane rotation around Ni that alternates along $c$. We find that such a $A_3^+$-mode instability has a generic character in the infinite-layer nickelates and can be tuned via either the effective $R$-site atom size or epitaxial strain.