Structure and properties of Ba8Ga16Ge30 clathrates by a novel synthesis method using CO gas reductive atmosphere

A novel synthesis method of Ba-Ga-Ge type-I clathrates is proposed. It enables us to use BaCO3 for one of the starting materials in place of Ba metals, which should be the greatest advantage of handling the materials, from the viewpoint of possible industrial application in the near future. By using this method, we have successfully produced the Ba8Ga16Ge30 compounds from the starting materials BaCO3, Ge and Ga. The synthesis reaction should occur simultaneously with the reduction reaction of BaCO3, which requires rather high reaction temperature beyond 1573 K. However, the resulting reactant still contains additional phases, such as Ge and Ba-Ga-O phases. The samples exhibited a positive value of the Seebeck coefficient from room temperature to 973 K, in spite of the fact that the Ba-Ga-Ge type-I clathrates were usually reported to show negative Seebeck coefficient.