Unraveling the origins of conduction band valley degeneracies in Mg2Si-1xSnx thermoelectrics

To better understand and enhance the thermoelectric efficiency of a new class of Mg-based thermoelectrics, using hybrid density-functional theory, we study the microscopic origins of valley degeneracies in the conduction band of the solid solution Mg2Si1-xSnx and its constituent components - namely, Mg2Si and Mg2Sn. In the solid solution of Mg2Si1-xSnx, the Mg sublattice and Si/Sn sublattice are expected to undergo either tensile or compressive strain. Interestingly, both tensile strain of Mg2Si and compressive strain of Mg2Sn enhance the conduction band valley degeneracy or band convergence, which has been strongly speculated as the electronic origin of the enhanced Seebeck coefficient in the Mg2Si1-xSnx system. We also consider finite-temperature electronic band structures of these systems to account for high temperature effects. Our results clearly highlight and demonstrate the role of sublattice strain in the band valley degeneracy observed in Mg2Si1-xSnx .