A Complete High-to-Low spin state Transition of Trivalent Cobalt Ion in Octahedral Symmetry in SrCo0.5Ru0.5O3-δ

The complex metal oxide SrCo0.5Ru0.5O3-delta 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 Co3+ ion is purely high-spin at room temperature, which is unique for a Co3+ 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) angstrom, 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 K beta 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 Co3+ 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 Co3+ ion in an octahedral coordination.