Thermoelectric properties of sintered clathrate compounds Sr8GaxGe46-x with various carrier concentrations

Polycrystalline samples of Sr8GaxGe46-x clathrates with varied nominal Ga contents were prepared by powder metallurgy to produce n-type samples with carrier concentration n of 1.8x10(20)-1.1x10(21) cm(-3). Their thermoelectric properties were measured in the temperature range from 300 to 900 K. Their Seebeck coefficient and electrical conductivity were found to be typical for a heavily doped semiconductor, and varied systematically with their carrier concentration. The maximum value of the figure-of-merit ZT was 0.62 at 800 K for n=5.6x10(20) cm(-3). Other following properties were also measured or estimated, and their relationships with the thermoelectric properties are discussed. The band gap of the samples was estimated to be 0.4-0.5 eV; their effective mass was an almost constant value of 3.1m(e) at room temperature, and this value decreases with increasing temperature; and their electron Hall mobilities were 8-13 cm(2) V-1 s(-1) at room temperature, increasing slightly with decreasing carrier concentration. (C) 2006 American Institute of Physics.