Dynamical structural instability and its implications for the physical properties of infinite-layer nickelates
Chengliang Xia, Jiaxuan Wu, Yue Chen, Hanghui Chen
Abstract
We use first-principles calculations to find that in infinite-layer nickelates $R{\mathrm{NiO}}_{2}$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R=\mathrm{La}\text{\ensuremath{-}}\mathrm{Sm}$. For late lanthanide elements $R=\mathrm{Eu}\text{\ensuremath{-}}\mathrm{Lu}$, an imaginary phonon frequency appears at $A=(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$ 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 ``${\mathrm{NiO}}_{4}$ square.'' Special attention is given to two borderline cases: ${\mathrm{PmNiO}}_{2}$ and ${\mathrm{SmNiO}}_{2}$, 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, $R{\mathrm{NiO}}_{2}$ in the $I4/mcm$ structure has a substantially reduced Ni ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ bandwidth, a smaller Ni $d$ occupancy, a ``cleaner'' Fermi surface with a lanthanide $d$-derived electron pocket suppressed at the $\mathrm{\ensuremath{\Gamma}}$ point, and a decreased critical ${U}_{\text{Ni}}$ 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 ``${\mathrm{NiO}}_{4}$ square'' rotation provides a tuning knob to render $R{\mathrm{NiO}}_{2}$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.