Simultaneous Realization of Single-Crystal-Like Electron Transport and Strong Phonon Scattering in Polycrystalline SrTiO3–xHx

Simultaneous realization of low thermal conductivity (κ) and high electronic conductivity (σ) has been a challenging issue in developing high-performance thermoelectric bulk materials. SrTiO₃ has been expected as an environmentally benign thermoelectric material, but the intrinsically high κ and low σ across the grain boundaries (GBs) restrict their performance. We recently tackled this obstacle by applying hydride anion (H^–) substitution, namely, SrTiO_3–xH_x. In this paper, in order to clarify the H^– substitution effect on electron and phonon scattering, we analyzed the temperature dependence of electronic and thermal transport properties of the SrTiO_3–xH_x bulks with different H^– concentrations x = 0.057–0.216. The carrier mobility was dominated by electron–electron scattering and impurity scattering; however, the GBs did not disturb the electron transport, and thus, the high σ approaching to 10⁴ S/cm at 4 K was realized in SrTiO_3–xH_x bulks. The H^– substitution concurrently suppressed the κ from 9.7 W/(mK) of SrTiO₃ to 5.0 W/(mK) of the SrTiO_3–xH_x bulk with x = 0.216 at room temperature. The phonon scattering by elastic strain field due to the largely different chemical bonding state of Ti-(O,H) bonds is found to be prominent for reducing κ. The H^– substitution approach would provide a possibility for developing environmentally benign thermoelectric oxides by the simultaneous realization of single-crystal-like electron transport and strong phonon scattering.