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

Abstract

Strontium cobalt oxide (SrCoO2.5) has recently attracted increasing attention as their 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 the thermopower is an excellent measure for analyzing the EF in SrCoO2.5 epitaxial films. The lattice mismatch caused grain size reduction, which induced a slight increase in the oxidation state of the Co ions due to additional adsorbed oxygen. Although X-ray spectroscopy analyses revealed the difference of the oxidation state of the Co ions among the samples are small, an unusually large change in thermopower from +330 μV K −1 (lattice-matched) to −185 μV K −1 (lattice-mismatched) is observed from the samples due to shifts in the EF to lower energy side. The present results demonstrate the excellent sensitivity of thermopower measurement for analyzing the location of EF in the electronic structure of SrCoO2.5 in the practically usable environment.

Solid-State Electrochemical Protonation of SrCoO2.5 into HxSrCoO2.5 (x = 1, 1.5, and 2)

“…Finally, ∼30 nm thick indium tin oxide (ITO) film was deposited as the top electrode at room temperature. Details of the PLD condition of each layer have been published elsewhere. ,,,,− …”

Solid-State Electrochemical Thermal Transistors with Strontium Cobaltite–Strontium Ferrite Solid Solutions as the Active Layers

“…Generally, when the Fermi level is closer to the band edge, the absolute value of the partial differential of the density of states (DOS) at the Fermi level becomes larger, the absolute value of its thermal power increases, and σ becomes smaller. 19 The relationship between log σ and x was used to calculate κ ele of the SrCo 1−x Fe x O 3 layers by assuming that the Wiedemann−Franz law holds. 20 This law is given as κ ele = L•σ•T, where L is the Lorentz number (2.44 × 10 −8 W Ω K −2 ).…”

A Switchable One‐Compound Diode

“…An important point to elucidate is the mechanism of the pnp‐transition in Ag 18 Cu 3 Te 11 Cl 3 . The Seebeck coefficient is defined by the Mott equation: [ 17,18 ] $$\[S\, = \, - \frac{{{\pi ^2}}}{3}\frac{{k_{\rm{B}}^2T}}{e}{\left[ {\frac{1}{n}\frac{{dn\left( E \right)}}{{dE}} + \frac{1}{\mu }\frac{{d\mu \left( E \right)}}{{dE}}} \right]_{E = {E_{\rm{F}}}}}\]$$where S , k B , e , n , and µ are the Seebeck coefficient, Boltzmann constant, electron charge, charge carrier concentration, and carrier mobility, respectively. This equation may be understood as the partial derivative of the density of states (DOS) analyzed at the Fermi energy.…”

“…An important point to elucidate is the mechanism of the pnptransition in Ag 18 Cu 3 Te 11 Cl 3 . The Seebeck coefficient is defined by the Mott equation: [17,18] 3…”