Simultaneous Realization of Single-Crystal-Like Electron Transport and Strong Phonon Scattering in Polycrystalline SrTiO_3–<i>x</i>H_<i>x</i>

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, SrTiO3–xHx. 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 SrTiO3–xHx 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 SrTiO3–xHx bulks. The H– substitution concurrently suppressed the κ from 9.7 W/(mK) of SrTiO3 to 5.0 W/(mK) of the SrTiO3–xHx 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.