High-temperature thermoelectric studies of A11Sb10 (A = Yb, Ca)

Large samples (6-8 g) of Yb11Sb10 and Ca11Sb10 have been synthesized using a high-temperature (1275-1375 K) flux method. These compounds are isostructural to Ho11Ge10, crystallizing in the body-centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The structure consists of antimony dumbbells and squares, reminiscent of Zn4Sb3 and filled Skutterudite (e.g., LaFe4Sb12) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (p), Seebeck coefficient (a), thermal conductivity (k), and thermoelectric figure of merit (zT) from room temperature to at minimum 975 K are presented for A(11)Sb(10) (A = Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to similar to 1200 K. Both A(11)Sb(10) (A = Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (similar to 10mW/cm K for both Yb11Sb10 and Ca11Sb10) that can be attributed to the complex crystal structure. Yb11Sb10 is a poor metal with relatively low resistivity (1.4 m Omega cm at 300 K), while Ca11Sb10 is a semiconductor suggesting that a gradual metal-insulator transition may be possible from a Ca11-xYbxSb10 solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT. zT. (c) 2007 Elsevier Inc. All rights reserved.