Thermoelectric properties of the Zintl phases Yb₅M₂Sb₆ (M = Al, Ga, In)

Zintl compounds with chemical formula Yb₅M₂Sb₆ (M = Al, Ga, and In) form one of two known A₅M₂Pn₆ structure types characterized by double chains of corner-linked MPn₄ tetrahedra bridged by Pn₂ dumbbells. High temperature electronic and thermal transport measurements were used to characterize the thermoelectric properties of Yb₅M₂Sb₆ compounds. All samples were found to exhibit similar high p-type carrier concentrations, low resistivity and low Seebeck coefficients in agreement with the band structure calculations. These results, combined with previous studies, suggest that Yb₅M₂Sb₆ compounds are semimetals (i.e., they lack an energy gap between the valence and conduction bands), in contrast to the semiconducting alkaline earth (Ca, Sr, Ba) and Eu based A₅M₂Sb₆ compounds. Yb₅M₂Sb₆ compounds have very low lattice thermal conductivity, comparable to other closely related A₅M₂Sb₆ and A₃MSb₃ phases. However, due to the semimetallic behaviour, the figure of merit of investigated samples remains low (zT < 0.15).