One Cation Makes a Difference: Structure–Thermoelectric Interplay in Pseudo–Rock Salt Intermetallic Eu _{5– <i>x</i>} A _<i>x</i> 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 Eu5–xYbxAl3Sb6 and Eu5–x–ySrxYbyAl3Sb6 were synthesized by flux methods and their structures determined by single-crystal X-ray diffraction, confirming monoclinic C2/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 Eu5Al4Sb6 and its Sr- and Yb-substituted solid solutions, along with the pseudoquinary phase Eu2.5Sr2Yb0.5Al4Sb6, is presented. Substituting Eu2+ with the more ionic Sr2+ enhances mobility and increases the magnitude of the Seebeck coefficient, while the more covalent Yb2+ 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.