Correlation between structure and mixed ionic–electronic conduction mechanism for (La_1−xSr_x)CoO_3−δ using synchrotron X-ray analysis and first principles calculations

Abstract

The mechanism of mixed ionic-electronic conduction (MIEC) for (La 1Àx Sr x )CoO 3Àd (LSC) (x ¼ 0.00-0.50) was investigated by experimental as well as theoretical studies which included Rietveld refinements, maximum entropy method (MEM) analysis, X-ray absorption spectroscopy, and first principles calculations (FPCs). The correlations among MIEC, structural parameters, i.e., the bond length, the bond angle, electron density, and the spin configuration, were discussed from the practical results. Moreover, these correlations were verified by FPC, and interpreted by the band structure and the total energy of LSC. The electronic conductivity was affected by the Co-O-Co bond angle, electron densities, and the spin state. In particular, the spin transition from the low to high spin state was accompanied by the cross-over from semiconductor to metallic at x ¼ 0.15. Concerning the oxide ion diffusion, LSC has large anisotropic atomic displacement parameters of the oxide ion sites, in the vertical direction of the Co-O bond, which associate with the high oxide ion diffusion. The total energy of LSC during the migration of the oxide ion was also calculated with varying Sr content, lattice parameters, and symmetries by using FPC. Consequently, the symmetry of LSC is more effective to reduce the activation energy of oxide ion diffusion than Sr content and lattice parameters.

Synthesis, sintering, and thermoelectric properties of the solid solution La1–xSr x CoO3±δ (0 ≤ x ≤ 1)

“…Therefore, the mixed valence of cobalt increases as a function of x, with a slope change for x > 0.15. These results of the valence of cobalt and the oxygen content as a function of x are similar to the previous studies estimated by various methods such as iodometric titration, Rietveld refinement, and thermogravimetric analysis [37,47,49,54,55]. Nevertheless, for x values greater than 0.9, a discontinuity in the variation of the content and mixed cobalt valence is observed.…”

“…This result points out that the unit cell elongates along the c-axis with increasing cation substitution content. This lattice parameter variation is similar to that observed in La 1-x Sr x CoO 3 obtained in previous works [47][48][49]. The…”

The Effect of Ni Doping on the Performance and Electronic Structure of LSCF Cathodes Used for IT-SOFCs

“…9,10,31,38 Indeed, the presence of Co(IV) has been reported by several authors also for LSCF and BSCF at different Co/Fe ratios. [34][35][36][37]28 Due to the large edge shift shown in Figure 6 (∼4 eV), it can be concluded that Co(III) and Co(IV) present in the structure are mostly reduced to Co(II), while evidence of the mere reduction of Co(IV) to Co(III) is not recognizable. Remarkably, only a negligible energy shift (<0.5 eV), measured in the same temperature range of unpromoted catalysts (see Figure 6), is observed for the sample LSCF08-Ni.…”

Improvement on the electrochemical performance by morphology control of nanostructured La0.6Sr0.4CoO3−δ-Gd0.1Ce0.9O1.95 cathodes for IT-SOFC

“…This forbidden transition is possible due to the hybridization between the Co-3d and O-2p orbitals, though some researchers have found evidence for quadrupolar transition nature. 33,35,57,58 In any case, it is clear that its features are closely related to the electronic conductivity properties of LSC and that it is strongly affected by the crystal field. 58,59 The reader is invited to consult the ESI, † for a more detailed discussion about this topic.…”

“…''Most of the studies of LSC, however, did not discuss the correlations among MIEC, crystal structure parameters, electron densities, band structures, and spin configurations, quantitatively.'' 35 Some works that focused the attention on the spin state properties at low or room temperature, made assumptions on cobalt's oxidation state from oxygen vacancy concentration or changes in the pre-edge-energy region of the Co-K edge XANES. [31][32][33] On the other hand, Takeda et al stated that the electrochemical activity of LSC (adsorption, diffusion, dissociation and reduction of oxygen and delivering of oxygen anions to the electrolyte phase, GDC, for instance) would partially rest on the ability of Co atoms for changing their oxidation state according to environment conditions and the electrochemical potential in a cell.…”