Thermoelectric Zintl phases with ultralow thermal conductivity: synthesis, structural characterization, and transport properties of Ca10AlSb9 and Ca10CdSb9

Abstract

Two new Zintl antimonides, Ca₁₀MSb₉ (M = Al and Cd), have been synthesized and characterized for the first time and their thermoelectric performance has been evaluated without additional optimization. The two phases can be readily synthesized either by Sn-flux or direct solid-state reactions. The compounds are isotypic and crystallize in a tetragonal crystal system with the space group P4₂/mnm (no. 136). The crystal structure is highly complex with significant structural disorder—both occupational and positional. For its careful elucidation, both single-crystal X-ray diffraction methods and Rietveld refinements of powder X-ray diffraction data for polycrystalline bulk samples were required. Despite the significant disorder, the structures retain their charge balance and these new Zintl phases are shown to be intrinsic semiconductors with a bandgap of about 0.5 eV. High-temperature transport measurements reveal remarkably low thermal conductivity (0.6–0.7 W m⁻¹ K⁻¹) in the temperature range 323 K to 573 K, which is even lower than that of the well-known thermoelectric Zintl material Yb₁₄MnSb₁₁. Additionally, Seebeck coefficient measurements showed a very high value for Ca₁₀AlSb₉, approaching 350 μV K⁻¹ at 573 K. Therefore, we speculate that with proper systematic work and further optimization, these and potentially other Zintl compounds from this extended family can show excellent thermoelectric performance.