A Complete High-to-Low spin state Transition of Trivalent Cobalt Ion in Octahedral Symmetry in SrCo<sub>0.5</sub>Ru<sub>0.5</sub>O<sub>3‑δ</sub>

The complex metal oxide SrCo<sub>0.5</sub>Ru<sub>0.5</sub>O<sub>3‑δ</sub> possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co<sup>3+</sup> ion is purely high-spin at room temperature, which is unique for a Co<sup>3+</sup> in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund’s-rule exchange due to a relatively large mean Co–O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co<sup>3+</sup> absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high-, intermediate-, and low-spin states as functions of the metal–oxygen bond length for a Co<sup>3+</sup> ion in an octahedral coordination.