Lattice normal modes and electronic properties of the correlated metal LaNiO₃

We use density functional theory calculations to study the lattice vibrations and electronic properties of the correlated metal LaNiO₃. To characterize the rhombohedral-to-cubic structural phase transition of perovskite LaNiO₃, we examine the evolution of the Raman-active phonon modes with temperature. We find that the A_1g Raman mode, whose frequency is sensitive to the electronic band structure, is a useful signature to characterize the octahedral rotations in rhombohedral LaNiO₃. We also study the importance of electron-electron correlation effects on the electronic structure with two approaches that go beyond the conventional band theory [local spin density approximation (LSDA)]: the local spin density+HubbardU method (LSDA+U) and hybrid exchange-correlation density functionals that include portions of exact Fock exchange. We find that the conventional LSDA accurately reproduces the delocalized nature of the valence states in LaNiO₃ and gives the best agreement with the available experimental data on the electronic structure of LaNiO₃. Based on our calculations, we show that the electronic screening effect from the delocalized Ni 3d and O-2p states mitigates the electronic correlations of the d7 Ni cations, making LaNiO₃ a weakly correlated metal.