Spin-State Transition in LaCoO3 and Related Materials

Abstract

Of the several inorganic systems that exhibit spin-state transitions, LaCoO3 and related cobaltates represent an important category of oxides exhibiting a transition from the low-spin (LS) state to a state of higher spin with increasing temperature. It was first considered that the transition was from the LS ((1)A(1)) to the high-spin (HS, T-5(2)) state and a variety of investigations were performed on this transition by employing magnetic susceptibility, Mossbauer spectroscopy, NMR spectroscopy and other measurements. These studies not only showed the evolution of the high-spin state with temperature but also the ordering of the two spin states and other related phenomena. The spin-gap energy in LaCoO3 is smaller than the charge-gap energy. The transition temperature varies depending on the rare earth ion in the LnCoO(3) series. In recent years, it has been demonstrated that the spin-state transition in LaCoO3 occurs initially from the LS state to the intermediate spin (IS, T-3(1)) state rather than to the HS state with increase in temperature. The intermediate spin-state Co3+ is a Jahn-Teller (JT) ion. The spin-state transition is therefore associated with lattice distortion, which is readily studied by infrared spectroscopy. Raman spectroscopy yields valuable information on the spin-state transition. Electronic structure calculations have been performed and the results verified experimentally by photon emission spectroscopy and other techniques. Recent studies indicate that it may be necessary to employ a three spin-state (LS-IS-HS) model rather than a two spin-state (LS-IS) model to fully explain the observed transition. Several theoretical models have been proposed to explain the spin-state transition in LaCoO3. These include the singlet-triplet transition model, the two-sublattice model, and the two-phonon model. The effect of hole doping on the spin-state transition has been examined in compounds like La1-xSrxCoO3. In this article, we discuss the various experimental and theoretical studies of LaCoO3 and related cobaltates.