BaBiCoRuO (0.0 ≤ ≤ 0.6) Hexagonal Double-Perovskite-Type Oxides as Promising p-Type Thermoelectric Materials.

A new series of BaBiCoRuO (0.0 ≤ ≤ 0.6) hexagonal double perovskite oxides have been synthesized by a solid-state reaction method by substituting Ba with Bi. The polycrystalline materials are structurally characterized by the laboratory X-ray diffraction, synchrotron X-ray, and neutron powder diffraction. The lattice parameters are found to increase with increasing Bi doping despite the smaller ionic radius of Bi compared to Ba. The expansion is attributed to the reduction of Co/Ru-site cations. Scanning electron microscopy further shows that the grain size increases with the Bi content. All BaBiCoRuO (0.0 ≤ ≤ 0.6) samples exhibit p-type behavior, and the electrical resistivity (ρ) is consistent with a small polaron hopping model. The Seebeck coefficient () and thermal conductivity (κ) are improved significantly with Bi doping. High values of the power factor (PF ∼ 6.64 × 10 W/m·K) and figure of merit ( ∼ 0.23) are obtained at 618 K for the = 0.6 sample. These results show that Bi doping is an effective approach for enhancing the thermoelectric properties of hexagonal BaBiCoRuO perovskite oxides.