New insight into the material parameter B to understand the enhanced thermoelectric performance of Mg<inf>2</inf>Sn<inf>1-x-y</inf>Ge<inf>x</inf>Sb<inf>y</inf>

Abstract

Historically, a material parameter B incorporating weighted mobility and lattice thermal conductivity has guided the exploration of novel thermoelectric materials. However, the conventional definition of B neglects the bipolar effect which can dramatically change the thermoelectric energy conversion efficiency at high temperatures. In this paper, a generalized material parameter B∗ is derived, which connects weighted mobility, lattice thermal conductivity, and the band gap. Based on the new parameter B∗, we explain the successful tuning of the electron and phonon transport in Mg2Sn1-x-yGexSby, with an improved ZT value from 0.6 in Mg2Sn0.99Sb0.01 to 1.4 in Mg2Sn0.73Ge0.25Sb0.02. We uncover that the Ge alloying approach simultaneously improves all the key variables in the material parameter B∗, with an ∼25% enhancement in the weighted mobility, ∼27% band gap widening, and ∼50% reduction in the lattice thermal conductivity. We show that a higher generalized parameter B∗ leads to a higher optimized ZT in Mg2Sn0.73Ge0.25Sb0.02, and some common thermoelectric materials. The new parameter B∗ provides a better characterization of material's thermoelectric transport, particularly at high temperatures, and therefore can facilitate the search for good thermoelectric materials.