The importance of the Mg–Mg interaction in Mg3Sb2–Mg3Bi2 shown through cation site alloying

Abstract

Herein we study the effect alloying Yb onto the octahedral cite of Te doped Mg₃Sb_1.5Bi_0.5 has on transport and the material's high temperature stability. We show that the reduction in mobility can be well explained with an alloy scattering argument due to disrupting the Mg_octahedral–Mg_tetrahedral interaction that is important for placing the conduction band minimum at a location with high valley degeneracy. We note this interaction likely dominates the conducting states across n-type Mg₃Sb₂–Mg₃Bi₂ solid solutions and explains why alloying on the anion site with Bi isn't detrimental to Mg₃Sb₂'s mobility. In addition to disrupting this Mg–Mg interaction, we find that alloying Yb into the Mg₃Sb₂ structure reduces its n-type dopability, likely originating from a change in the octahedral site's vacancy formation energy. We conclude showing that while the material's figure of merit is reduced with the addition of Yb alloying, its high temperature stability is greatly improved. This study demonstrates a site-specific alloying effect that will be important in other complex thermoelectric semiconductors such as Zintl phases.