Quantum chemical study of Co<Superscript>3+</Superscript> spin states in LaCoO<Subscript>3</Subscript>

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO<Subscript>3</Subscript>. The main concern is to calculate the energy level ordering of different spin states of Co<Superscript>3+</Superscript>, which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T=5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S=0) ground state is separated from the first excited high-spin (HS, S=2) state by a gap >100 meV, while the intermediate-spin (IS, S=1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co<Superscript>3+</Superscript> ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200 <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\div$</EquationSource> </InlineEquation> 300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010