Direct measurement of Co-ion spin state transitions in Ca₃Co₄O₉ using variable-temperature electron energy-loss spectroscopy

Previous studies of Ca₃Co₄O₉ suggested that the abrupt changes in the magnetic susceptibility and electrical resistivity at ∼420 K can be attributed to Co spin state transitions. In this letter, we study the possible transitions above 420 K by variable-temperature Z-contrast imaging and electron energy-loss spectroscopy. We find that there is no observable change in the structure and Co valence state upon in situ heating to 500 K, compared to room temperature. However, an intensity decrease of the prepeak in the O K-edge near edge fine structure indicates a Co³⁺-ion spin state transition has occurred from a low to an intermediate spin state.