Ytterbium silicide-based thermoelectric materials have attracted attention because they exhibit large power factor owing to large absolute values of their Seebeck coefficient as well as high electrical conductivity. In this study, in order to figure out the reason that ytterbium silicides exhibit such high power factor, we synthesized Yb_1-xLu_xSi₂ and characterized the Yb valence and the Seebeck coefficients as a function of temperature from 20K to 300K. The Yb valences in Yb_1-xLu_xSi₂ estimated by XAFS spectrum indicated mixed valences with 2.7 and 2.8 for x＝0.0 and 0.5, respectively. The minimum value of the Seebeck coefficient was －57.6 µVK^－1 at 127K, which could be caused by a f- electron contribution due to the mixed valence.

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We evaluated Bi-substitution effects of Ge₂Sb_2－xBi_xTe₅ (x＝0, 0.1, 0.3, 0.5) bulk materials on the crystal structures and low-temperature thermoelectric properties in the temperature range of 100-300 K. When the substitution amount of Bi increased from x＝0 to 0.5, the ratio of cubic to hexagonal structure changed from 79: 21 to 67:33 in wt%, resulting in increase in the ratio of hexagonal structure in the sample. With increasing x, Seebeck coefficient S would be mainly influenced by the increase in the phase fraction of the hexagonal structure, and electrical resistivity ρ and thermal conductivity κ would be affected by the alloy scattering. As a result, the maximum dimensionless figure of merit zT at 300 K decreased from 0.07 (x＝0) to 0.02 (x＝0.5) because decrease in S²ρ^－1 exceeded decrease in κ.

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