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

We have studied the electronic structure of ${\mathrm{BaCoO}}_{3}$ using soft x-ray absorption spectroscopy at the Co ${L}_{2,3}$ and O $K$ edges, magnetic circular dichroism at the Co ${L}_{2,3}$ edges, and 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 $3{d}^{5}$ state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling also plays 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 $3{d}^{5}$ ion away from the ${J}_{\mathrm{eff}}=1/2$ state. The sign of this crystal field is such that the ${a}_{1g}$ orbital is doubly occupied, explaining the absence of a Peierls transition in this system, which consists of chains of face-sharing ${\mathrm{CoO}}_{6}$ octahedra. Moreover, with one hole residing in ${e}_{g}^{\ensuremath{\pi}}$, the presence of an orbital moment and strong magnetocrystalline 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 ${\mathrm{BaCoO}}_{3}$.