Hydride Synthesis and Thermoelectric Properties of Type-I Clathrate K₈E₈Ge₃₈ (E = Al, Ga, In)

Type I clathrates of the composition K₈E₈Ge₃₈ (E = Al, Ga, In) were prepared via the reaction of KH with E and Ge and thermoelectric properties measured in order to compare to K₈Al₈Si₃₈, a promising thermoelectric material. The structures were confirmed with Rietveld refinement of powder diffraction patterns obtaining lattice parameters of 10.7729(2) Å, 10.7469(5) Å, and 10.9975(6) Å for E = Al, Ga, and In, respectively. Samples of K₈E₈Ge₃₈ with E = Al and Ga were consolidated via spark plasma sintering for property measurements and determined to be 94.2% and 81.4% dense, respectively. K₈In₈Ge₃₈ showed significant decomposition after sintering with both elemental In and Ge present in the powder diffraction pattern. The thermoelectric properties of K₈E₈Ge₃₈ (E = Al, Ga) from 300-10 K were measured on sintered pellets. K₈Al₈Ge₃₈ was found to have a Seebeck coefficient, electrical resistivity, and thermal conductivity of -35.8 μV/K, 2.56 mΩ·cm, 1.37 W/m·K at 300 K, respectively. K₈Ga₈Ge₃₈ was found to be a compensated semiconductor with a Seebeck coefficient, electrical resistivity, and thermal conductivity of 4.19 μV/K, 1080 mΩ·cm, and 1.05 W/m·K at 300 K, respectively. The resistivity of K₈Al₈Ge₃₈ is 46 times lower than K₈Al₈Si₃₈ which has a Seebeck coefficient of -90.0 μV/K and thermal conductivity of 1.77 W/mK at 300 K, suggesting that a solid solution of K₈Al₈Si_38-xGe _x has potential for optimal thermoelectric performance.