Thermoelectric properties of p-type Ba8Ga16Ge30 type-I clathrate compounds prepared by the vertical Bridgman method

•P-type Ba8Ga16Ge30 type-I clathrates were prepared by the Bridgman method.•The optimal thermoelectric zT of the Ba8Ga16Ge30 clathrate reaches 1.10 at 823 K.•Doubling in electrical conductivity with slight decrease in thermopower was found.

P-type polycrystalline Ba8Ga16+xGe30−y type-I clathrate compounds were grown from melts by means of the vertical Bridgman method. The composition of melts was varied to obtain ingots with three different Ga/Ge ratios. The exact composition and crystal structure of the clathrates were respectively determined by electron probe microanalysis and X-ray diffraction, while the Seebeck coefficient, electrical conductivity and thermal conductivity were measured from room temperature up to 873 K. It was found that the sample with the Ga/Ge ratio of 0.58 has the optimal dimensionless figure-of-merit zT of 1.10 at 823 K. The Ba8Ga16+xGe30−y clathrate compound with the optimal zT possesses a moderate carrier concentration but the highest carrier mobility among the three conditions. While the Seebeck coefficient of the Ba8Ga16+xGe30−y sample with the optimal zT is lower than literature data for single crystal, the increases of its electrical conductivity in this study plays the key role in maintaining zT with a comparable thermal conductivity.