One Cation Makes a Difference: Structure–Thermoelectric Interplay in Pseudo–Rock Salt Intermetallic Eu _{5– x} A _x Al ₃ Sb ₆ (A = Sr and Yb)

Polar intermetallics are an emerging class of thermoelectric materials whose electronic properties can be finely tuned by cation chemistry. Single crystals of Eu 5– x Yb x Al 3 Sb 6 and Eu 5– x–y Sr x Yb y Al 3 Sb 6 were synthesized by flux methods and their structures determined by single-crystal X-ray diffraction, confirming monoclinic C 2/ m symmetry and an electron count near 3.5 e – per atom, consistent with polar intermetallic classification. The Al content in these phases can be increased from 3 to 4. A comparative study of polycrystalline synthesized Eu 5 Al 4 Sb 6 and its Sr- and Yb-substituted solid solutions, along with the pseudoquinary phase Eu 2.5 Sr 2 Yb 0.5 Al 4 Sb 6, is presented. Substituting Eu 2+ with the more ionic Sr 2+ enhances mobility and increases the magnitude of the Seebeck coefficient, while the more covalent Yb 2+ drives the system metallic, lowering Seebeck values but improving zT to 0.8 at 873 K. The quinary phase further suppresses bipolar conduction, delaying the high-temperature downturn observed in both ternary solid solutions. Across all compositions, thermal conductivities remain exceptionally low (<1 W m –1 K –1 ), enabling promising figures of merit. These results highlight how ionic versus covalent A-site substitution can serve as a powerful lever to control scattering, band overlap, and transport in polar intermetallics, opening design pathways parallel to those of the benchmarked half-Heusler phases and PbTe.