High-spin and low-spin mixed state in LaSrCoO₄: An ab initio study

Spin state is an important issue for many cobaltates, and an intermediate spin (IS) state having a half-filled e_g orbital may well be expected for a Co³⁺ ion in a CoO₆ octahedron with a remarkable tetragonal distortion. Here the single-layered perovskite cobaltate LaSrCoO₄, which has a notable tetragonal elongation, is investigated for its spin state and electronic structure, through a set of local-spin-density approximation plus Hubbard U (LSDA+U) calculations including also the multiplet effect and spin-orbit coupling. Counterintuitively, our calculations evidence that the IS state is not the ground state and it would, even if being so, give rise to a wrong ferromagnetic half-metallic solution. We find that a strong band hybridization significantly suppresses a multiplet energy splitting of the IS state. Instead, a high-spin (HS) and low-spin (LS) mixed state turns out to have the lowest total energy among all possibly combined spin states. Moreover, the mixed HS+LS ground state well accounts for the experimental paramagnetic insulating behavior, the effective magnetic moment, and the observed optical spectral features. We also predict that LaSrCoO₄ in the mixed HS+LS ground state has a sizeable out-of-plane orbital moment and a local lattice distortion, which would motivate experimental studies.