Breathing distortions in the metallic, antiferromagnetic phase of LaNiO₃

I study the structural and magnetic instabilities in LaNiO₃ using density functional theory calculations. From the non-spin-polarized structural relaxations, I find that several structures with different Glazer tilts lie close in energy. The Pnma structure is marginally favored compared to the R3c structure in my calculations, suggesting the presence of finite-temperature structural fluctuations and a possible proximity to a structural quantum critical point. In the spin-polarized relaxations, both structures exhibit the ↑0↓0 antiferromagnetic ordering with a rock-salt arrangement of the octahedral breathing distortions. The energy gain due to the breathing distortions is larger than that due to the antiferromagnetic ordering. These phases are semimetallic with small three-dimensional Fermi pockets, which is largely consistent with the recent observation of the coexistence of antiferromagnetism and metallicity in LaNiO₃ single crystals by Guo et al. [Nat. Commun. 9, 43 (2018)].