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Abstract:
To investigate the influence of chemical pressure on the e(g) orbital occupation and the spin state of Co3+ ions, which are closely related to electrochemical reaction of catalysts for energy storage applications, we have prepared a new material Sr1.7Ca0.3CoO3F, which was previously predicted to possess an intermediate-spin (IS) state with e(g)(1) configuration. Our experimental Co-L-2,L-3 X-ray absorption spectrum of Sr1.7Ca0.3CoO3F was very similar to those of the high-spin (HS) Co3+ references Sr2CoO3F and Sr2CoO3Cl indicating an HS Co3+ ground state. The HS-Co3+ ground state in Sr1.7Ca0.3CoO3F was further confirmed by using O-K XAS spectra and theoretical simulation of the Co-L-2,L-3 XAS spectra with configuration-interaction cluster calculations. We present theoretical energy diagrams of three spin states for Co3+ ions in Sr2CoO3F 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 angstrom) and the out-of-plane Co displacement (<0.18 angstrom) are critical conditions for obtaining the IS-Co3+ ground state.
