Bonding mediated coupling of electron band mass and phonon group velocity: an effective strategy to improve the thermoelectric performance of solid solutions

Abstract

Solid solutions often demonstrate superior thermoelectric (TE) performances. A judicious choice of alloy composition can result in band convergence and increased phonon scattering, which will enhance the TE figure of merit (zT) value. However, an often overlooked fact is the alloying effect on chemical bonds and its implications for TE performance. Herein, we demonstrate that alloying-/doping-induced alterations in chemical bonding can simultaneously affect band mass , phonon group velocity (v) and deformation potential (Ξ). Herein, a series of n-type Mg_2.2(Si_0.3Sn_0.7)_1−xBi_x (x = 0−0.025) compositions were synthesized, and information on their electronic and phonon band structures was extracted from experiments (modeling the measured TE properties) and density functional theory (DFT) calculations. The parallel occurrence of band flattening, acoustic phonon softening, and lowering of deformation potentials was observed from experimental and DFT results. The connection between these seemingly unrelated effects could be traced to variations in the hybridization and antibonding orbital population induced by alloying and doping. Changes in bond hybridization were a manifestation of chemical substitution and resulted in the observed band flattening. Bond alteration with alloying and raising of the Fermi level position caused by doping contributed to the increased anti-bonding character and affected phonon group velocity and deformation potential. The cumulative effect of these changes was shown to overshadow the band convergence effect and was the primary cause of the high zT_max in Mg₂Si_0.3Sn_0.7. The study, therefore, reveals that bond regulation by a careful choice of alloying and doping elements can increase the anti-bonding character and enable the mixing of different orbitals, thus opening a new pathway for achieving performance enhancement in TE materials.