Synthesis and thermoelectric properties of silicon clathrates Sr8AlxGa16-xSi30 with the type-I and type-VIII structures

Nominal composition Sr(8)AlxGa(16-x)Si(30) (0 <= x <= 16) samples were prepared by powder metallurgy. The samples with x = 0-7 exhibited the type-I clathrate structure (Pm3n, No. 223), while the samples with x = 8-13 almost exhibited the type-VIII clathrate structure (1 (4) over bar 3m, No. 217). These samples possessed the electrical conductivities and Seebeck coefficients typical of n-type degenerated semiconductors. The results on their thermoelectric properties indicate that the type-VIII SrAlGaSi clathrate would be a more efficient thermoelectric material than the typed SrAlGaSi clathrate. For example, the type-VIII Sr8Al9Ga7Si30 sample had a maximum power factor of 11 mu W cm(-1) K-2 at 1000 K and a room temperature Hall mobility of 24 cm(2) V-1 S-1, while those of the type-I Sr8Al6Ga10Si30 sample were 7 mu Wcm(-1) K-2 and 5 cm(2) V-1 s(-1), respectively. (C) 2008 The Japan Society of Applied Physics.