Mg vacancy and dislocation strains as strong phonon scatterers in Mg₂Si_1−xSb_x thermoelectric materials

Mg₂Si based solid solutions have unique advantages in the direct thermal to electrical energy conversion due to environmentally friendly and abundant constituent elements, and high thermoelectric performance. Further enhancing figure of merit zT of this materials system lies in the reduction of the relatively high lattice thermal conductivity. Alloying by high content of aliovalent Sb (>10%) in Mg₂Si can greatly suppress the lattice thermal conductivity, lower than conventional Mg₂(Si,Sn) solid solutions, due to the enhanced phonon scattering from Mg vacancy and concomitant strains. In this work, detailed microstructure observation for Sb alloyed Mg₂Si reveal the existence of dense dislocations, around which the strip-like defects are viewed with isotropic strain. By introducing dislocations and vacancies as the additional phonon scattering sources, the model calculation can well match the experimental lattice thermal conductivity. The present work offers a comprehensive understanding of the role of vacancies and concomitant strains in reducing thermal conductivity, and may open a new venue for zT enhancement.