Yb_(14)MnSb_(11) is a p-type high-temperature thermoelectric material with operational temperatures as high as 1273 K. Rare-earth (RE) substitution into this phase has been shown to increase the melting point further while also decreasing the sublimation rate. Solid solutions of 3+ RE elements with Yb~(2+) in Yb_(14)MnSb_(11) have shown to have increased stability against oxidation. Ce is an abundant RE element, and the substitution of Ce~(3+) on the Yb~(2+) sites should increase the thermoelectric efficiency of the material due to a decrease in carrier concentration. Samples of Yb_(14–x )Ce_(x )MnSb_(11) (x ∼ 0.4) were synthesized using ball milling, followed by annealing and consolidation via spark plasma sintering. The systematic addition of a small increase of excess Mn and the resulting compositions were investigated. Small amounts of impurities in the samples, such as Yb_(2)O_(3) and Mn, are correlated with negative attributes in the resistivity data. Hall effect measurements revealed a reduced carrier concentration of ∼44 at 600 K over Yb_(14)MnSb_(11), and adjusting the stoichiometry toward Yb_(13.6)Ce_(0.4)MnSb_(11) leads to increases in resistivity and the Seebeck coefficient with a reduction in thermal conductivity. Yb_(13.6)Ce_(0.4)MnSb_(11) shows an improved average zT _(avg) = 0.80 when compared to Yb_(14)MnSb_(11) (0.71) and no degradation when exposed to ambient air for 77 days at room temperature. Thermogravimetric analysis of air oxidation shows that Yb_(13.6)Ce_(0.4)MnSb_(11) and Yb_(14)MnSb_(11) do not oxidize until 700 K.
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