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 A(5)M(2)Pn(6) structure types characterized by double chains of corner-linked MPn(4) tetrahedra bridged by Pn(2) dumb-bells. 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 semi-metals (i.e., they lack an energy gap between the valence and conduction bands), in contrast to the semi-conducting alkaline earth (Ca, Sr, Ba) and Eu based A(5)M(2)Sb(6) compounds. Yb5M2Sb6 compounds have very low lattice thermal conductivity, comparable to other closely related A(5)M(2)Sb(6) and A(3)MSb(3) phases. However, due to the semimetallic behaviour, the figure of merit of investigated samples remains low (zT < 0.15).