Synthesis, structure, and high temperature thermoelectric properties of Yb11Sb9.3Ge0.5

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 Yb11Sb10 were synthesized using a molten Sn-flux technique. Single crystal X-ray diffraction data were obtained and resulted in a composition of Yb11Sb9.3Ge0.5 which was verified by microprobe. Yb11Sb9.3Ge0.5 is isostructural to Ho11Ge10, crystallizing in a body-centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb11Sb9.3Ge0.5 are a = 11.8813(4), c = 17.1276(13) angstrom with a volume of 2417.8(2) angstrom(3). These parameters correlate well with the structural refinement of previously published Yb11Sb10. The structure consists of 16 isolated Sb3- anions, 8 Sb-2(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 Sb-4(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. Yb11Sb9.3Ge0.5 shows p-type behavior increasing from similar to 22 mu V/K at room temperature to similar to 31 mu V/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.