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The n-type polycrystalline materials of Ba 8 Ga 16 Ge 30 bulks have been successfully synthesized via the high-temperature and high-pressure (HPHT) method, and their crystal structure has been characterized using the X-ray diffraction. Through the thermoelectric properties measurements and microstructure studies, we know that the improvement of the dimensionless thermoelectric figure-of-merit (ZT) benefits from the low lattice thermal conductivity due to the increased low, medium, and high wavelength phonon scattering by grain boundaries, nanostructures, nanoscale domains, and lattice defects, which are ubiquitous in the synthetic bulk material. As the temperature increases, the power factor (PF) obviously increases, and the thermal conductivity evidently decreases. A minimum thermal conductivity of 0.71 W m −1 K −1 was reached at 773 K for Ba 8 Ga 16 Ge 30 synthesized at 4 GPa. A good ZT value 1.14 was achieved, which was attributed to not only the enhanced PF but also the relative low thermal conductivity.

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