Spin-orbit coupling and crystal-field distortions for a low-spin 3d5 state in BaCoO3

Abstract

We have studied the electronic structure of BaCoO3 using soft x-ray absorption spectroscopy at the Co-L2,3 and O-K edges, magnetic circular dichroism at the Co-L2,3 edges, as well as valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent 3d 5 state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling plays also an important role for the magnetism of the system. At the same time, a trigonal crystal field is present with sufficient strength to bring the 3d 5 ion away from the J ef f = 1/2 state. The sign of this crystal field is such that the a1g orbital is doubly occupied, explaining the absence of a Peierl's transition in this system which consists of chains of face-sharing CoO6 octahedra. Moreover, with one hole residing in the e π g , the presence of an orbital moment and strong magneto-crystalline anisotropy can be understood. Yet, we also infer that crystal fields with lower symmetry must be present to reproduce the measured orbital moment quantitatively, thereby suggesting the possibility for orbital ordering to occur in BaCoO3.

Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

“…11) decreases with increasing valence state of the TM ion 47 . For Co 2+ , it is positive 48,49 ; for Co 3+ , it is close to zero 50 ; and for Co 4+ , it can be negative 27,28,51 . Consequently, the covalency increases, i.e., from rather ionic for Co 2+ , to highly covalent for Co 3+ , and further to Co 4+ , in which the holes mainly reside on the oxygen ligands rather than on the Co 27,28 .…”

“…The Co valence in these two cases can be estimated by constructing a weighted sum of the Li 2 Co 2 O 4 spectrum (blue line) and BaCoO 3 spectrum (cyan line, from Ref. 27 ), as shown in Fig. 2e, f. We find Co 3.2+ and Co 3.4+ valences for these two cases, respectively.…”

“…In Supplementary Fig. 4e, we show that there is a change and shift to higher energies of the Co spectrum when comparing the Li 2 Co 2 O 4 after the OER with the as-prepared Li 2 Co 2 O 4 , indicating an increase in the Co valence 27,36,37 . We also quantitatively analyzed the Co-L 2,3 spectra in terms of a weighted superposition of the theoretical LS Co 3+ and LS Co 4+ spectra, see Supplementary Fig.…”

“…For instance, the Co 4+ oxides BaCoO 3 (Ref. 27 ) and SrCoO 3 (Ref. 28 ) are negative charge-transfer systems, in which the 3d 5 configuration has a contribution of <10%, whereas the dominant configuration is 3d 6 L (L stands for the hole in the ligand O 2p states) with t 2g 5 e g 1 L eg for a low-spin state and t 2g 4 e g 2 L eg for an intermediate-spin state as shown in Ref.…”

“…Such O 2p holes can be directly studied by soft X-ray absorption spectroscopy (SXAS) at the O-K edge, especially for an edgeshared Co-O network, which presents a well-separated Co 3+ -and Co 4+ -related spectral feature in the spectra of Co 3+ /Co 4+ mixed valence oxides 29,30 . Thus, the O-K SXAS spectra can generally be used to explore the Co 4+ content via Co 3d-O 2p covalence 27,28 .…”