Thermoelectric properties of ternary and Al-containing quaternary Ru_1-xRe_xSi_y chimney-ladder compounds

The thermoelectric properties of ternary and Al-containing quaternary Ru_1-xRe_xSi_y chimney-ladder phases have been studied as a function of the Re concentration with the use of directionally solidified alloys. The Ru_1-xRe_xSi_y chimney-ladder phases exhibit n- and p-type semiconducting behaviors, respectively, at low and high Re concentrations, at which the X(=Si)/M(=Ru + Re) ratios are respectively, larger and smaller than those expected from the VEC (valence electron concentration) = 14 rule. The absolute values of both Seebeck coefficient and electrical resistivity increase as the extent of the deviation from the VEC = 14 rule increases, i.e. as the alloy composition deviates from that corresponding to the p-n transition (x ≈ 0.5), indicating that the carrier concentration can be controlled by changing the extent of compositional deviation from the ideal VEC = 14 composition. The highest values of the dimensionless figure of merit obtained are 0.47 for ternary (x = 0.60) and 0.56 for Al-containing quaternary alloys. The reasons for the systematic compositional deviation from the ideal VEC = 14 compositions observed for a series of chimney-ladder phases are discussed in terms of atomic packing.