Occurrence and Variation of Spin-State Transitions in La _1-xEu_xCoO₃ Cobaltates

Structural, magnetic, and Co K-edge X-ray absorption near-edge structure (XANES) spectroscopic studies for the stoichiometric La_1-xEu _xCoO₃ (0 ≤ x ≤ 1) system with trivalent Co ³⁺ are reported. The thermally-excited spin-state transition from low-spin (LS) to intermediate-spin (IS) state increases from T_s ∼ 105 K for LaCoO₃ (x = 0) to ∼140 K for x = 0.25, to ∼200 K for x = 0.5, and extrapolates to ∼295 K for EuCoO₃ (x = 1), where the IS state is a mixture of d⁶ (t_2g ⁵ e_g¹) and d⁷L (t_2g ⁵ e_g² with a ligand hole) with filled t _2g majority spin states. The small pre-edge feature observed in room temperature XANES for the system in the IS state is from the 1s-3d quadrupole transition, which is weakly allowed through the hybridization of Co 4p states with 3d states of the neighboring Co atoms. For LaCoO₃, the pre-edge can be fitted using three Gaussian peaks with energy separations ΔE(P ₂-P₁) = 2.0 eV and ΔE(P₃-P₂) = 1.5 eV, where P₁, P₂, and P₃ presumably correspond to the unfilled t_2g minority, e_g majority, and e_g minority spin states, respectively.