Synthesis, structure, and high temperature thermoelectric properties of Yb₁₁Sb_9.3Ge_0.5

Zintl phase compounds with large unit cells and complex anionic structures such as Yb₁₁Sb₁₀ hold potential for being good thermoelectric materials. Single crystals of Ge-doped Yb₁₁Sb ₁₀ were synthesized using a molten Sn-flux technique. Single crystal X-ray diffraction data were obtained and resulted in a composition of Yb ₁₁Sb_9.3Ge_0.5 which was verified by microprobe. Yb₁₁Sb_9.3Ge_0.5 is isostructural to Ho ₁₁Ge₁₀, crystallizing in a bodycentered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb ₁₁Sb_9.3Ge_0.5 are a = 11.8813(4), c = 17.1276(13) Å with a volume of 2417.8(2) ų. These parameters correlate well with the structural refinement of previously published Yb₁₁ Sb₁₀- The structure consists of 16 isolated Sb ^3- anions, 8 Sb₂^4- dumbbells, 2 Sb ₄^4- square planar rings and 44 Yb²⁺ cations. The Ge, doped in at 28 % occupancy, was found to be site specific, residing on the 2 Sb₄^4- square planar rings. Single crystal X-ray diffraction is most consistent with the site that makes up the square ring being less than fully occupied. The doped compound is additionally characterized by X-ray powder diffraction, differential scanning calorimetry and thermogravimetry. High temperature (300-1200 K) thermoelectric properties show that the doped compound has extremely low thermal conductivity (10-30 mW/cmK), lower than that of Yb₁₁Sb₁₀. Temperature dependent resistivity is consistent with a heavily doped semiconductor. Yb ₁₁Sb_9.3Ge_0.5 shows p-type behavior increasing from ∼22 μV/K at room temperature to ∼31 μV/K at 1140 K. The low value and the temperature dependence of the Seebeck coefficient suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.