Experimental and Theoretical Soft X‐Ray Absorption Study on Co3+ Ion Spin States in Sr2−xCaxCoO3F.

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 Sr_1.7Ca_0.3CoO₃F, which was previously predicted to possess an intermediate‐spin (IS) state with e_g¹ configuration. Our experimental Co‐L_2,3 X‐ray absorption spectrum of Sr_1.7Ca_0.3CoO₃F 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 Sr_1.7Ca_0.3CoO₃F 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 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 Å) and the out‐of‐plane Co displacement (<0.18 Å) are critical conditions for obtaining the IS‐Co³⁺ ground state. [ABSTRACT FROM AUTHOR]