Figure 4: Influence of lanthanides on electronic structure.

(a) Cobalt oxidation obtained by chemical titration (closed symbols) and by XAS at the Co K-edge (open symbols) plotted versus the oxygen content for (Ln_0.5Ba_0.5)CoO_3−δ (LnBaCo₂O_5+δ′ with δ′=1−2δ and Ln=Pr, Sm, Gd and Ho), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and LaCoO₃ (LCO). The oxygen vacancy concentration δ was directly calculated from the cobalt oxidation state x by δ=(3.5−x)/2. The error bars in the oxidation state obtained by chemical titration represent the s.d. from at least three independent measurements. The errors in the oxidation state obtained by XAS were calculated by taking the root of the squared s.e. obtained by linear regression (see Methods). (b) The computed O p-band centre versus the Fermi level for the nonstoichiometric (Ln_0.5Ba_0.5)CoO_3−δ double perovskites (having δ constrained to the values obtained from chemical titration) and stoichiometric (Ln_0.5Ba_0.5)CoO₃ in the constrained or relaxed structure (see Supplementary Information). (c) O K-edge XAS data of (Ln_0.5Ba_0.5)CoO_3−δ measured in the total electron yield mode. (d) Integrated intensities of the O K-edge pre-edge region normalized to oxygen content per formula unit and the nominal number of empty cobalt 3d states in both e_g and t_2g symmetry.