The result of our research is an unified description of the magnetic and electric transitions in cobaltites LnCoO3 (Ln = La, Y, rare earths), associated with thermally activated changes of the spin state of the octahedrally coordinated Co3+ ions, and the determination of the polaron character of the hole and electron carriers in doped cobaltites.
Oxide systems with cobalt ions in octahedral coordination attract attention of both the basic and applied research since. Ionic states of Con+ (n = 2, 3, 4) represent a unique situation in transition metal oxides because of the possibility of several 3dm (m = 7, 6, 5) electronic configurations. This is associated with a subtle balance between different energy contributions associated with the CoO6 octahedra - namely the crystal-field splitting (separation of 3d states into the t2g and eg sublevels), the on-site (electron-electron) Coulomb repulsion and the energy of charge transfer between transition metal and oxygen (the Co3d-O2p hybridization). In the case of Co3+ ion, the low spin (LS, t2g6eg0, S=0), intermediate spin (IS, t2g5eg1, S=1) or high spin (HS, t2g4eg2, S=2) states can be realized. Typical examples are the LaCoO3-derived perovskites that exhibit two transitions in dependence on temperature. The low-temperature transition is associated with a gradual excitation of Co3+ ions from the diamagnetic LS state to a paramagnetic (IS or HS) state, while the high-temperature transition is of the insulator-metal kind, associated also with a magnetic anomaly. The basic properties were described in early 1960‘s. However, there still remains a controversy about the character of the excited species and nature of the high-temperature metallic phase, despite large research effort.
Our work represents experimental and theoretical research of perovskite cobaltites LnCoO3 (Ln = La, Y, rare earth). Systematic calculations of the stability of different Co3+ spin states in the perovskite structure have proved that the transition in LaCoO3 at T = 100 K consists of a local excitation from the LS ground state to the close-lying HS state, and pointed to a strong repulsion between neighboring HS states. The analysis of magnetic susceptibility in LaCoO3 showed that the metallic state realized above the second transition (insulator-metal) is associated with a homogeneous phase with cobalt ions in intermediate-spin state (IS), which coexists with residual regions in the LS+HS mixture. Based on the electron structure calculations it was possible to relate the origin of the IS phase to a temperature activated electron exchange between the LS Co3+-HS Co3+ pairs. The present scenario of the two-level spin transition in LaCoO3, LS-LS/HS-IS, can be used also for other compounds LnCoO3, where the LS ground state is stabilized with decreasing size of Ln = Nd, Pr,...Dy, Y ions, and both transitions are shifted to higher temperatures, approach each other and merge finally.
The new model interprets the magnetic and electric behavior not only in the single-valent cobaltites LnCoO3, but also in the doped systems of a mixed Co3+/Co4+ or Co3+/Co2+ valency. The work done on hole- or electron-doped systems LaCo1-xMxO3 and DyCo1-xMxO3 (x = 0 - 0.05, M = Mg2+ and Ti4+) has shown that both kinds of carriers induce magnetic states on neighboring Co sites, originally in the diamagnetic LS Co3+ state. This forms a magnetic polaron of large total spin. Consistently with the above mentioned scenario, the polarons can be viewed as as droplets of the IS phase that move in the background of the low-temperature LS or LS/HS phases of undoped LnCoO3 and are finally dissolved in the high-temperature homogeneous IS phase of the host.
