Structural instabilities of infinite-layer nickelates from first-principles simulations

Rare-earth nickelates $R\mathrm{Ni}{\mathrm{O}}_{2}$ adopting an infinite-layer phase show superconductivity once La, Pr, or Nd is substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high-symmetry, perfectly symmetric, ${P}_{4}\text{/}mmm$ tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understanding of the role of chemical pressure or strain on lattice distortions that in turn could alter magnetic and electronic properties of the two-dimensional nickelates. Here, by performing a full analysis of the prototypical ${\mathrm{YNiO}}_{2}$ compound with first-principles simulations, we identify that these materials are prone to exhibit ${\mathrm{O}}_{4}$ group rotations whose type and amplitude are governed by the usual $R$-to-Ni cation size mismatch. We further show that these rotations can be easily tuned by external stimuli modifying lattice parameters such as pressure or strain. Finally, we reveal that H intercalation is favored for any infinite-layer nickelate member and pushes the propensity of the compounds to exhibit octahedra rotations.