Geometric effects in the infinite-layer nickelates

Geometric effects in the infinite-layer nickelates RNiO2 associated with the relative size of the R-site atom are investigated via first-principles calculations. We consider, in particular, the prospective YNiO2 material to illustrate the impact of these effects. Compared to LaNiO2, we find that the La -> 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 YNiO2 departs from the cupratelike 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 RNiO2 series. This implies a P4/mmm <-> I4/mcm structural transformation associated to an 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 an 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.