Experimental and Theoretical Soft X‐Ray Absorption Study on Co³⁺ Ion Spin States in Sr_2−xCa_xCoO₃F

Abstract

To investigate the influence of chemical pressure on the e g orbital occupation and the spin state of Co 3þ ions, which are closely related to electrochemical reaction of catalysts for energy storage applications, we have prepared a new material Sr 1.7 Ca 0.3 CoO 3 F, which was previously predicted to possess an intermediate-spin (IS) state with e g 1 configuration. Our experimental Co-L 2,3 X-ray absorption spectrum of Sr 1.7 Ca 0.3 CoO 3 F was very similar to those of the high-spin (HS) Co 3þ references Sr 2 CoO 3 F and Sr 2 CoO 3 Cl indicating an HS Co 3þ ground state. The HS-Co 3þ ground state in Sr 1.7 Ca 0.3 CoO 3 F was further confirmed by using O-K XAS spectra and theoretical simulation of the Co-L 2,3 XAS spectra with configuration-interaction cluster calculations. We present theoretical energy diagrams of three spin states for Co 3þ ions in Sr 2 CoO 3 F as a function of external pressure based on the crystal structures observed in a previous high-pressure study. We have also investigated the structural conditions that are required to stabilize the IS state in this effective local pyramidal structure theoretically, and found that the in-plane Co-O distance (<1.88 Å) and the out-of-plane Co displacement (<0.18 Å) are critical conditions for obtaining the IS-Co 3þ ground state.

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

“…Second, if the Co ion remains in the trivalent state when the CoO 5 pyramidal coordination is formed, then there will be not one electron transferred to the e g shell but two electrons. The associated high- spin (HS) state for the t 2g 4 e g 2 configuration is well documented in the literature for Co 3+ ions with pyramidal local coordination [42][43][44][45][46] . The spectrum of such an HS CoO 5 pyramidal system is given in Supplementary Fig.…”

Spin State and Spin‐State Transition of Co3+ Ion in BiCoO3

“…More spectroscopic evidence for the HS nature of Co 3þ in BiCoO 3 can also be found in the O-K XAS spectra due to the covalent mixing between Co 3d and O 2p. [4,6,17] The O-K edge XAS spectra of sandwiched (Figure 4c) and mixed (Figure 4d) BiCoO 3 together with Li 2 Co 2 O 4 (Figure 4a) and Sr 2 CoO 3 Cl (Figure 4b) are shown in Figure 4. Here, the spectral features in the 527-532 eV energy range represent transitions from the O core 1s level to the unoccupied 2p states.…”

“…[ 24 ] Thus, we observed a pure HS‐Co 3+ state in the samples prepared from the mixed method, which is fully consistent with the pyramidal local coordination possessing weak crystal field interaction. [ 6,17,24,39 ]…”

“…Among 3d transition metal oxides, perovskite-type cobalt oxides possess a unique feature: the changes in the orbital and spin state of Co 3þ ion manifest themselves in the competition between the low spin (LS) t 2g 6 , high spin (HS) t 2g 4 e g 2 , and even intermediate spin (IS) t 2g 5 e g 1 , which is the socalled spin state degree of freedom. [1,2] The interplays among the charge, orbital, and spin degrees of freedom generate a series of specific features of magnetic, electric, and structural properties, for example, spin state transitions upon decreasing temperature [3][4][5] or under high pressure, [6][7][8] giant magnetostriction, [9] spin blockade, [10] and considerable thermoelectric effects. [11,12] Moreover, it was also found recently that the high performance of widely used electrochemical cobalt oxide catalysts for water splitting attributed to the unity e g occupancy.…”

“…[13][14][15][16] Multiferroic BiCoO 3 has a PbTiO 3 -type structure but with much larger distortion, thus leading to a high c/a ratio of 1.267 and calculated polarization of 120 μC cm À2 . The Co ion in BiCoO 3 is pyramidally coordinated by five oxygen ions, which is similar to Sr 2 CoO 3 Cl [17] and Sr 2 CoO 3 F. [6] The PbTiO 3 -type structure and large distortion make BiCoO 3 a promising candidate as a parent compound to search for lead-free piezoelectric materials. For example, the morphotropic phase boundary phase has been observed in Bi(Co 1Àx Fe x )O 3 samples.…”