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 $R$NiO$_2$, 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 "NiO$_4$ square". Special attention is given to two borderline cases: PmNiO$_2$ and SmNiO$_2$, in which both the $P4/mmm$ structure and the $I4/mcm$ structure are local minima and the energy difference between the two structures can be fine-tuned by epitaxial strain. Compared to the $P4/mmm$ structure, $R$NiO$_2$ in the $I4/mcm$ structure has a substantially reduced Ni $d_{x^2-y^2}$ 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 $U_{\textrm{Ni}}$ 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 "NiO$_4$ square" rotation provides a tuning knob to render $R$NiO$_2$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.

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