Background: The electronic and geometrical properties of LaCoO₃, which has a closed shell ground state (S_z=0), are compared with the ones of KCoF₃, presenting a high spin (S_z=3/2) ground state. In an ideal fully ionic model, the Co occupancy in the two compounds is d⁶ (oxide) and d⁷ (fluoride).
Methods: The analysis of the density of states and of the charge distribution, the latter through a Mulliken analysis, shows that in the oxide the partially covalent character of the Co-O bonds deeply alters the ideal picture, and that not only the t_2g, but also the e_g subshell is occupied, with an overall d occupancy close to 7 electrons (one electron in the e_g subshell), very close to the fluoride occupancy.
Results and conclusion: The deformation with respect to the cubic aristotype (SG 221), down to the tetragonal (SG 123, 127 and 140), orthorhombic (SG 62) and rhombohedral (SG 167) symmetry, with a corresponding increase of the inner degrees of freedom (from 1 to 10) and size of the unit cell (from 5 to 20 atoms), is explored, which confirms that the octahedra remain essentially regular, with an energy gain along the full group-subgroup chain of only 4.6 mE_h (0.125 eV) per formula unit. In spite of the similar total occupancy of the d shell, the electronic configuration of Co in KCoF₃ is quite different, with a high spin (, with 5α and 2β electrons) ground state.