The effect of tm substitution on the thermoelectric performance of Yb₁₄MnSb₁₁

The Zintl phase, Yb₁₄MnSb₁₁, has been recently identified as a potential thermoelectric material because of its high figure of merit (zT) above 1000 K. Tuning the carrier concentration with La³⁺ provided an improved zT at high temperatures. Rare earth elements provide additional tuning through size, electron donor properties, and effects on bonding. The solubility of the rare earth, Tm, was explored to provide chemical pressure and enhance the Seebeck coefficient of Yb₁₄MnSb₁₁. Crystals were prepared via Sn flux synthesis with the compositional fluxes of Yb_14-xTm_xMnSb₁₁ (x = 0.3 (1), 0.5 (2), 0.7 (3)). The incorporation of Tm does not significantly alter the unit cell parameters of Yb₁₄MnSb₁₁. The content of Tm was probed with wavelength dispersive microprobe analysis and showed that the maximum amount of Tm is 0.44 with the composition being Yb_13.34(6)Tm_0.44(1)Mn_1.04(2)Sb_11.19(6). Single crystal compositions of Yb_14-xTm_xMnSb₁₁ (x = 0.29 (1), 0.40 (2), and 0.44 (3)) were measured. Samples were hot pressed and thermoelectric properties measured from 300-1200 K. The hot pressed pellets showed very similar Tm composition of approximately x = 0.4. All the samples show enhanced Seebeck coefficients and slightly higher electrical resistivity as compared with Yb₁₄MnSb₁₁, as expected for the addition of Tm³⁺ in this p-type compound. Total thermal conductivity is approximately the same as Yb₁₄MnSb₁₁ leading to a maximum zT of 0.81 at 1195 K for sample 2 with nominal composition Yb_13.35(4)Tm_0.44(1)Mn_1.027(6)Sb_11.18(4).