Unusually Large Thermopower Change from +330 to −185 μV K^–1 of Brownmillerite SrCoO_2.5

Strontium cobalt oxide (SrCoO2.5) has recently attracted increasing attention as its optoelectronic and magnetic properties can be widely controlled using electrochemical oxidation/protonation at room temperature in air. To utilize the versatile properties of SrCoO2.5, it is essential to evaluate the location of the Fermi energy (EF) in the electronic structure, which is sensitive to the oxidation state of the Co ions. Here, we show that thermopower is an excellent measure for analyzing the EF in SrCoO2.5 epitaxial films. The lattice mismatch causes grain size reduction, which induces a slight increase in the oxidation state of the Co ions due to additional adsorbed oxygen. Although X-ray spectroscopy analyses reveal that the difference of the oxidation state of the Co ions among the samples is small, an unusually large change in thermopower from +330 μV K–1 (lattice-matched) to −185 μV K–1 (lattice-mismatched) is observed in the samples due to shifts in the EF to the lower energy side. The present results demonstrate the excellent sensitivity of thermopower measurements for analyzing the location of EF in the electronic structure of SrCoO2.5 in a practically usable environment.