The role of simultaneous substitution of Cr and Ru on the thermoelectric properties of defect manganese silicides MnSiδ (1.73 < δ < 1.75)

• Defect manganese silicides show unusual low thermal conductivity.
• Beneficial Cr protects the low thermal conductivity and enhances the power factor.
• Ru is detrimental, as it reduces the charge carrier mobility.

Defect manganese silicides have been substituted with Cr and Ru, in order to study the role of these substitutions on the resulting resistivity (ρ), Seebeck (α) and Hall coefficients and thermal conductivity. The resistivity and Seebeck coefficient measurements suggest that these compounds are degenerate semiconductors. The Hall measurements indicate that the carrier concentration increases in Cr and Ru substituted compounds. The majority carriers are holes and the change of hole concentration is only marginal. While the hole concentration (p) of MnSi1.74 is ∼2.1 × 1021 cm−3, in heavily substituted Mn0.65Cr0.2Ru0.15Si1.74, p increases only to ∼4.5 × 1021 cm−3. The increase in p is presumably due to Si precipitation. One interesting feature is the low lattice thermal conductivity (κL) which is ∼2–2.5 W/m K at 300 K depending upon the composition. In addition, κL shows a weak temperature dependence. Because of the combination of high power factor (α2/ρ) ∼ 2 mW/m K2 and low thermal conductivity ∼2.5–2.7 W/m K, Mn0.95Cr0.5Si1.74 and Mn0.97Cr0.3Si1.74 exhibit highest ZT of 0.6 at 850 K. For other compounds, a moderate ZT value of 0.3–0.5 is realized at 850 K. In Ru substituted compounds, the ZT is not improved beyond 0.5, due to the decrease of charge carrier mobility.