Origin of the unique thermoelectric transport in Mg3(Sb,Bi)2: absence of d-orbital bonding in crystal cohesion

Abstract

The Zintl phase compound Mg₃(Sb,Bi)₂ has attracted much research interest, owing to its excellent thermoelectric performance comparable to n-type Bi₂Te₃-based materials. Among the family of AMg₂X₂ (A = Mg, Ca, Sr, Ba, Yb; X = Sb, Bi), Mg₃(Sb,Bi)₂ exhibits very exceptional thermoelectric behavior. It is the only member being experimentally n-type doped, and its lattice thermal conductivity is significantly lower than those of the other members, such as CaMg₂Sb₂ and YbMg₂Bi₂. Band structure calculations revealed that the conduction band minimum of AMg₂X₂ is located at the high symmetry ‘M’ point, whereas Mg₃(Sb,Bi)₂ is an exception. Here, we present a systematic theoretical study to clarify the origin of the anomalous behavior of Mg₃(Sb,Bi)₂. It is revealed that the thermoelectric transport of the other AMg₂X₂ compounds is dominated by the strong p–d bonding between the ‘X’ site and ‘A’ site. In Mg₃(Sb,Bi)₂, however, the d orbital bonding is absent due to the small period number of Mg. The weak ‘A–X’ site bonding in Mg₃(Sb,Bi)₂ is proven to be responsible for all the unusual properties. Based on the new understanding, BaMg₂SbBi is proposed as a new candidate for n-type compounds for its relatively weak p–d bonding. Our work resolves the puzzling behaviors of Mg₃(Sb,Bi)₂ and provides an insight into the thermoelectric transport of the whole AMg₂X₂ family.