Electronic structure and thermoelectric properties of noble metal clathrates:: Ba8M6Ge40(M = Cu, Ag, Au)

Electronic structure and thermoelectric properties of noble metal clathrates Ba8M6Ge40(M = Cu, Ag, An) were calculated by using a first principle based method to discuss prospect of high performance thermoelectric materials. The calculated band structures show that these clathrate compounds are degenerate semiconductors with 1) type carrier. The band gaps become narrow by noble metal doping. The band gap narrowing is caused by the valence band lifting due to anti-bonding nature between Ge and M, and conduction band widening due to frameworkguest atom coupling via Ba orbitals at 6d sites. The thermoelectric power of Ba8M6Ge40 increases with temperatures monotonically and the magnitude is smaller than 100 mu V/K under 1000K. The carrier concentration of Ba(8)M(6)GC(40) is a little larger for the high performance thermoelectric materials. The carrier concentration is controlled by varying the composition of Ba8AuxGe46-x. The optimized carrier concentration in Ba8AuxGe46-x was obtained as 4.4 x 10(26) m(-3) (p type) and 6.4 x 10(26) m(-3) (n type), respectively.