Intermetallic Zintl compounds of the Ca₁₄AlSb₁₁ (14-1-11) structure type are important for their small bandgap semiconducting behavior and exceptionally low thermal conductivity. A new member of this structure type, Eu₁₄GaAs_11, containing zero-dimensional tetrahedral [GaAs₄]^9- anions, has been synthesized. This compound crystallizes in the tetragonal I4₁/acd space group, similar to other 14-1-11 compounds. Eu₁₄GaAs₁₁ is a semiconductor with a bandgap of 0.61 eV, as calculated using the Goldsmid-Sharp formula. Electronic transport measurements indicate a high Seebeck coefficient of 232 µV/K at 321 K, peaking at 424 µV/K at 713 K. The electrical resistivity is particularly high due to low carrier concentrations. However, the compound's notable strengths include its stability at high temperatures and its ultra-low thermal conductivity of 0.59 W m⁻¹ K⁻¹ at room temperature, with minimal electronic contribution, making it even lower than that of other high-performing Zintl phases. Given its low thermal conductivity and high Seebeck coefficient, Eu₁₄GaAs₁₁ presents potential for further optimization by adjusting the carrier concentration to enhance its thermoelectric performance.

Polycrystalline Eu14GaAs11 was synthesized through high-energy ball milling and high-temperature solid-state reactions. All procedures were conducted in an inert atmosphere glove box. High-purity elemental sources, such as Eu ingot (Stanford Advanced Materials, 99.99%), As chips (Johnson Matthey Chemicals, 99.9999%), and Ga (Alfa Aesar, 99.99999%), were used for synthesis. Binary precursors, EuAs and GaAs, were prepared by precisely weighing the necessary stoichiometric amounts of elements with a precision balance and milling them in a 65-cm³ stainless-steel grinding vial with two stainless steel balls of 12.7 mm diameter, using a SPEX 8000D Mixer/Mill for 30 minutes. To ensure homogeneous mixing, the mixture was scraped with a chisel and milled again for an hour. The homogeneous powder was then sealed in a Ta tube and enclosed within an evacuated quartz tube. EuAs was synthesized by annealing at 850 °C for 12 hours, and GaAs at 650 °C for 12 hours. Eu14GaAs11 was prepared from the binary precursors and elements in stoichiometric ratios, following the same procedures but annealing at 1100 °C for 96 hours.

The powder was pressed into pellets using a Dr. Sinter Junior Spark Plasma Sintering system (Fuji Electronic Industrial Co., Ltd.) under argon gas. The applied force increased from 5 kN to 9 kN, and the pellets were sintered at 900 °C for 30 minutes. The density of the pellets exceeded 98%, as determined by Archimedes' principle.

Caution: Arsenic-containing phases may produce toxic arsine gas during reactions. The quartz tube was handled in a fume hood, while the metal tubes were managed inside the glove box. Proper personal protective equipment was worn at all times.

Phase characterization involved using a Bruker D8 ECO ADVANCE powder X-ray diffraction (PXRD) diffractometer on zero-background off-axis quartz plates, with Cu Kα radiation, over 2θ angles of 20°−80° with a step size of 0.02°. A portion of the densified pellet was mounted on an epoxy resin disk and polished with 1000-grit sandpaper, then further polished with a wheel and 1 μm colloidal diamond suspension. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed using a Thermo Fisher Quattro ESEM with a Bruker Quantax EDX detector at 20 kV to analyze phase homogeneity and elemental distribution. The EDS composition averages data from ten points across the pellet.

Thermogravimetry and differential scanning calorimetry (TG/DSC) analyses assessed thermal stability, conducted from room temperature to 900 °C under an argon flow of 50 mL/min, with a heating rate of 10 K/min.

Transport properties were measured by determining thermal diffusivity with a Netzsch LFA 475 Microflash under argon flow, with pellet surfaces coated with graphite to improve laser absorption. Total thermal conductivity was calculated via D × Cp × r, where D is thermal diffusivity, r is density, and Cp, approximated using the Dulong-Petit law, is heat capacity.

The Seebeck coefficient was measured with a custom instrument under controlled low pressure (300 Torr) in a nitrogen atmosphere over 300 K to 775 K. Electrical resistivity and Hall measurements used an in-house setup based on the van der Pauw method. Hall coefficient measurements employed currents of 15-20 mA and a magnetic field of 1 T.