Effects of Sc and Y substitution on the structure and thermoelectric properties of Yb₁₄MnSb₁₁

Yb₁₄MnSb₁₁ is the most efficient bulk p-type thermoelectric material for high temperature applications. Materials with Y and Sc substitutions in Yb₁₄MnSb₁₁ were made both in Sn-flux and by ball milling. These small 3+ rare earth (RE) cations were introduced with the goal of providing chemical pressure on the structure. The RE³⁺ cation is smaller than Yb²⁺ and also donates one additional electron to this p-type semiconductor. In Yb_14-x RE _x MnSb₁₁ (RE = Sc, Y) the maximum x was about 0.5. X-ray diffraction experiments on the single crystals obtained from Sn-flux showed that Sc preferentially substitutes for Yb(1) and Yb(3), and decreases the size of the unit cell by about 0.3%. Y substitutes on all Yb sites and increases the size of the unit cell by about 0.2%. Samples with Yb_14-x RE _x MnSb₁₁ (x~0.3) were prepared via powder metallurgy and spark plasma sintering for transport and thermal conductivity measurements. Electron microprobe of the Sc-substituted sample showed small regions (≤1 μm) containing greater amounts of Sc, and X-ray powder diffraction of the ball milled Sc sample could be fitted as phase pure Yb_14-x Sc _x MnSb₁₁. Y-substituted samples showed larger regions of excess Y in electron microprobe, and small amounts of Yb₄Sb₃ in X-ray powder diffraction. The Sc sample has slightly reduced carrier concentration over optimized Yb₁₄MnSb₁₁, while the Y samples have even lower carrier concentrations. These carrier concentrations lead to comparable resistivity to Yb₁₄MnSb₁₁ in the Sc-substituted material, and higher resistivities in the Y-substituted material. All materials had similar Seebeck coefficients that slightly exceed Yb₁₄MnSb₁₁ at high temperatures, with the Sc-substituted sample having the highest despite having a higher carrier concentration. Sc-substituted samples also had a slightly higher thermal conductivity over the Y-substituted samples, which had comparable thermal conductivity to Yb₁₄MnSb₁₁. The zT values of the Sc and Y substituted samples are similar (zT_1000K~0.8), however below that of Yb₁₄MnSb₁₁ due to the compensation of Seebeck and resistivity.