Dynamical structural instability and its implication on the physical properties of infinite-layer nickelates

We use first-principles calculations to find that in infinite-layer nickelates RNiO₂, the widely studied tetragonal P4/mmm structure is only dynamically stable for early lanthanide elements R = La-Sm. For late lanthanide elements R = Eu-Lu, an imaginary phonon frequency appears at A = (π,π,π) point. For those infinite-layer nickelates, condensation of this phonon mode into the P4/mmm structure leads to a more energetically favorable I4/mcm structure that is characterized by an out-of-phase rotation of "NiO₄ square". Special attention is given to two borderline cases: PmNiO₂ and SmNiO₂, in which both the P4/mmm structure and the I4/mcm structure are local minimums, and the energy difference between the two structures can be fine-tuned by epitaxial strain. Compared to the P4/mmm structure, RNiO₂ in the I4/mcm structure has a substantially reduced Ni d_x²-y² bandwidth, a smaller Ni d occupancy, a "cleaner" Fermi surface with a lanthanide-d-derived electron pocket suppressed at Γ point, and a decreased critical U_Ni to stabilize long-range antiferromagnetic ordering. All these features imply enhanced correlation effects and favor Mott physics. Our work reveals the importance of structure-property relation in infinite-layer nickelates, in particular the spontaneous "NiO₄ square" rotation provides a tuning knob to render RNiO₂ in the I4/mcm structure a closer analogy to superconducting infinite-layer cuprates.