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

TL;DR

This study uses hybrid density-functional theory to investigate how strain affects conduction band valley degeneracies in Mg2Si1-xSnx thermoelectrics, revealing strain-induced band convergence as a key factor for enhanced thermoelectric performance.

Contribution

It provides a microscopic understanding of how sublattice strain influences valley degeneracies in Mg2Si1-xSnx, advancing the design of more efficient thermoelectric materials.

Findings

Tensile strain in Mg2Si enhances valley degeneracy.

Compressive strain in Mg2Sn also enhances valley degeneracy.

Finite-temperature effects confirm strain's role in band structure.

Abstract

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 .