One cation makes a difference: structure-thermoelectric interplay in pseudo-rock salt intermetallic Eu5-<em>x</em>A<em>x</em>Al3Sb6 (A = Sr and Yb)

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 the monoclinic C2/m symmetry. The Al content in these phases can be increased from 3 to 4 by metallurgical mixing of the elements to form polycrystalline powders. A comparative study of polycrystalline synthesized Eu5Al4Sb6 and its Sr- and Yb-substituted solid solutions, along with the pseudo-quinary 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. Methods The datasets (Powder X-ray diffraction and Thermoelectric property measurements) for the phase pure powders are provided. Compositions are provided below. Synthesis: Phase pure powders of composition Eu5.08Al4Sb4, Eu5Al4Sb6, Eu5-xAxAl4Sb6 (A = Sr: x = 0.5, 2.5, 5; A = Yb: x = 0.5, 1, 1.5, 2, 5) and Eu2.5Sr2Yb0.5Al4Sb6 Powder X-ray Diffraction (PXRD): PXRD data were collected at room temperature in air on ground single crystals and bulk powders of both solid solutions using a Bruker D8 Advance Eco diffractometer with Cu Kα radiation, 1000 W (40 kV, 25 mA), over 2θ = 15–85°, with a step size of 0.02°, and a scan rate of 1 s per step. Polycrystalline samples and ground single crystals were prepared on a zero-background holder using isopropanol. Thermal Conductivity Thermal Diffusivity (D) was measured on thin (1.19 to 1.2 mm thick) slices of Eu5-xAxAl4Sb6 (A = Sr and Yb) pellets using a Netzsch LFA 457 Microflash under a flow of high-purity Ar with a polished piece of Zr ribbon wrapped around the sample holder to act as an oxygen catcher. The thermal conductivity (κ) was determined using the equation: κ = D x ρ x Cp. The Dulong-Petit heat capacity value, Cp, was calculated from 3R/atom (R = gas constant). The pellets’ density (ρ) was measured in replicate using the Archimedes method with toluene as the liquid. All samples were > 95% of their theoretical crystallographic densities, calculated using the Arrhenius method. Electrical Resistivity and Seebeck A Linseis LSR-3 instrument was used to measure resistivity and the Seebeck coefficient employing the four-probe method from 350 K to 800 K under a He atmosphere. The instrument was calibrated with a constantan standard before use. The sample geometries were bar-shaped (10.5 mm x 4 mm x 2 mm) and polished before measurements with 8 mm probes. Multiple samples were measured to ensure reproducibility, and the data were cross-checked with measurements taken at Northwestern University. At Northwestern, electrical resistivity and Hall effect data were measured using a home-built Hall instrument. This set-up uses a four-point van der Pauw resistivity measurement with molybdenum leads and a current of 100 mA. Seebeck coefficient data were collected using a home-built two-probe Seebeck instrument with chromel/Nb thermocouples.