Thermoelectric properties of the Zintl phases Yb_5M_2Sb_6 (M = Al, Ga, In)

Zintl compounds with chemical formula Yb_5M_2Sb_6 (M = Al, Ga, and In) form one of two known A_5M_2Pn_6 structure types characterized by double chains of corner-linked MPn_4 tetrahedra bridged by Pn_2 dumbbells. High temperature electronic and thermal transport measurements were used to characterize the thermoelectric properties of Yb_5M_2Sb_6 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 Yb_5M_2Sb_6 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 A_5M_2Sb_6 compounds. Yb_5M_2Sb_6 compounds have very low lattice thermal conductivity, comparable to other closely related A_5M_2Sb_6 and A_3MSb_3 phases. However, due to the semimetallic behaviour, the figure of merit of investigated samples remains low (zT < 0.15).