Dynamical structural instability and a new crystal-electronic structure of superconducting infinite-layer nickelates

We use first-principles calculations to find that in superconducting 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. The 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 their energy difference can be tuned via epitaxial strain. Compared to the P4/mmm structure, RNiO₂ in the I4/mcm structure has a substantially reduced Ni dx2−y2  bandwidth, a smaller Ni d occupancy, a “cleaner” Fermi surface with less contribution from lanthanide element d orbitals and a decreased critical U_Ni to stabilize long-range antiferromagnetic ordering. All these features favor Mott physics and render RNiO₂ in the I4/mcm structure a closer analogy to superconducting infinite-layer cuprates.