Design of tetragonal tungsten bronze-type thermoelectric oxides with low lattice thermal conductivity enabled by complex cations and polar nanoregions

Reduction of lattice thermal conductivity has been a major challenge in developing high-performance oxide thermoelectrics. In this study, we propose a new strategy to reduce lattice thermal conductivity by introducing complex cation disorder and polar nanoregions (PNRs) as multiscale phonon-scattering centers. As a model system, we synthesized a new series of tetragonal tungsten bronze-type thermoelectric oxides (Na_1/2RE_1/2)_0.6Ba_0.4Nb₂O₆ (RE = Y, La, Gd, Yb). Dielectric measurements suggested the presence of PNRs in all compositions. After annealing in H₂/N₂, (Na_1/2La_1/2)_0.6Ba_0.4Nb₂O₆ showed the highest electrical conductivity, resulting in the highest power factor of 233 μW m⁻¹ K⁻² at 1000 K. All compositions exhibited low thermal conductivity around 2 W m⁻¹ K⁻¹ with glass-like weak temperature dependence. Consequently, a maximum ZT of 0.078 was obtained at 773 K for (Na_1/2La_1/2)_0.6Ba_0.4Nb₂O₆. These results suggest that the simultaneous introduction of complex cations and PNRs is an effective strategy for suppressing lattice thermal conductivity in oxide thermoelectrics.