Energetic Stability and Thermoelectric Property of Alkali-Metal-Encapsulated Type-I Silicon-Clathrate from First-Principles Calculation

Possible combinations of alkali metal guest atoms and substitutional group-13 atoms in type-I Si clathrate and their thermoelectric properties were investigated using first-principles calculations. All alkali metals could be encapsulated as the guest element into a Si-46 cage, and either Al or Ga was suitable for a substitutional atom. From the formation energy, possible clathrate compositions were selected as K8Al8Si38, K8Ga8Si38, Rb8Al8Si38, Rb8Ga8Si38, Cs8Al8Si38 and Cs8Ga8Si38. The thermoelectric properties of these compositions were calculated as functions of temperature and carrier density, using the Boltzmann transport equation and the calculated band energy. The obtained dependences of the Seebeck coefficient and electrical conductivity on the carrier density were discussed from the viewpoint of band structure. The thermoelectric properties were optimized to maximize ZT for each composition by controlling the carrier density. ZT approximate to 0.75 was predicted as the highest ZT value for hole-doped Cs8Ga8Si38.