Synthesis, Characterization, and Low Temperature Transport Properties of Eu_11-xYb_xCd₆Sb₁₂ Solid-Solution Zintl Phases

Eu_11-xYb_xCd₆Sb₁₂ Zintl solid solutions have been prepared by tin flux reaction by employing the elements Eu/Yb/Cd/Sb/Sn in the ratio 11 - x_p:x_p:6:12:30, where x_p is an integer less than 11 representing the preparative amount of Eu (11 - x_p) and Yb (x_p). Efforts to make the Yb compositions for x exceeding ∼3 resulted in structures other than the Sr₁₁Cd₆Sb₁₂ structure type. The crystal structures and compositions were determined by single-crystal and powder X-ray diffraction and wavelength-dispersive X-ray analysis measurements. The title solid-solution Zintl compounds crystallize in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr₁₁Cd₆Sb₁₂ structure type (Pearson symbol mC58), and the lattice parameters decrease with increasing ytterbium content. Single crystal X-ray diffraction shows that Yb atoms are not randomly distributed in the Eu sites but have a site preference which can be attributed to size effects. The influence of the rare earth (RE) metal sites on thermal and electronic properties of RE₁₁Cd₆Sb₁₂ solid solutions has been studied by measuring their thermoelectric properties from 5 to 300 K after consolidation by either spark plasma sintering (SPS) or hot pressing (HP). Electron microprobe analysis reveals that some of the rare earth metal is lost during SPS; as a result pellets formed through SPS have lower electrical resistivity by an order of magnitude due to increased hole-charge carrier concentrations. While the carrier concentration increases, the mobility decreases due to deficiencies in Eu content. Refinement of powder X-ray diffraction shows that Eu loss is mainly from the Eu1 crystallographic site, which has a unique coordination suggesting that this site plays a key role in the transport properties of RE₁₁Cd₆Sb₁₂.