Hydride Anion Substitution Boosts Thermoelectric Performance of Polycrystalline SrTiO3 via Simultaneous Realization of Reduced Thermal Conductivity and High Electronic Conductivity

The development of environmentally benign thermoelectric materials with high energy conversion efficiency (ZT) continues to be a long-standing challenge. So far, high ZT has been achieved using heavy elements to reduce lattice thermal conductivity (κ_lat). However, it is not preferred to use such elements because of their environmental load and high material cost. Here a new approach utilizing hydride anion (H^?) substitution to oxide ion is proposed for ZT enhancement in thermoelectric oxide SrTiO₃ bulk polycrystals. Light element H^? substitution largely reduces κ_lat from 8.2 W/(mK) of SrTiO₃ to 3.5 W/(mK) for SrTiO_3?xH_x with x = 0.216. The mass difference effect on phonon scattering is small in the SrTiO_3?xH_x, while local structure distortion arising from the distributed Ti?(O,H) bond lengths strongly enhances phonon scattering. The polycrystalline SrTiO_3?xH_x shows high electronic conductivity comparable to La-doped SrTiO₃ single crystal because the H^? substitution does not form a grain boundary potential barrier and thus suppresses electron scattering. As a consequence, SrTiO_3?xH_x bulk exhibits maximum ZT = 0.11 at room temperature and the ZT value increases continuously up to 0.22 at T = 657 K. The H^? substitution idea offers a new approach for ZT enhancement in thermoelectric materials without utilizing heavy elements.