Unusual mixed spin-state of Co3+ in the ground state of LaSrCoO4: Combined high-pressure and high-temperature study

The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L2,3 edge of LaSrCoO4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively