Impact of Oxygen Stoichiometry on the Thermoelectric Properties of Bi 2 Sr 2 Co 2 O y Thin Films.

In this study, we present a comprehensive analysis of the thermoelectric (TE) properties of highly c -axis-oriented thin films of layered misfit cobaltates Bi 2 Sr 2 Co 2 O y . The films exhibit a high c -axis orientation, facilitating precise measurements of electronic transport and TE properties along the a - b crystallographic plane. Our findings reveal that the presence of nearly stoichiometric oxygen content results in high thermopower with metallic conductivity, while the annealing of the films in a reduced oxygen atmosphere eliminates their metallic behavior. According to the well-established Heike's limit, the thermopower tends to become temperature independent when the thermal energy significantly exceeds the bandwidth, which provides a rough estimation of charge carrier density by using the Heike's formula. This observation suggests that the dominant contribution to the thermopower comes from the narrow Co-t 2g bands near the Fermi energy. Our study demonstrates that the calculated thermopower value using Heike's formula, based on the Hall electron density of the Bi 2 Sr 2 Co 2 O y thin films at 300 K, aligns well with the experimental results, shedding light on the intriguing TE properties of this family of layered cobaltate oxide films.