Synergy of Valence Band Modulation and Grain Boundary Engineering Leading to Improved Thermoelectric Performance in SnTe

SnTe is a lead-free and promising p-type thermoelectric material in an intermediate temperature range (500-900 K), but its intrinsic performance is poor due to excess cation vacancies and relatively large band offset between L and Σ valence bands. In this study, we alloyed Sb2Te3 into the SnTe matrix and successfully promoted the valence band convergence as supported by the calculated two-band Pisarenko curve; we further doped excess Mg and obtained a significantly reduced lattice thermal conductivity ∼0.44 W m-1 K-1 at 623 K. Scanning transmission electron microscopy (STEM) reveals that a large amount of Mg-rich nanoprecipitates concentrate at SnTe grain boundaries and stay coherent with the SnTe lattice, which is believed to be responsible for the large reduction of lattice thermal conductivity. Through the synergetic effects of valence band modulation and grain boundary engineering, a decent ZTmax of ∼1.14 at 723 K and an impressively large ZTavg of ∼0.67 (323-773 K) were achieved simultaneously in the (Sn0.95Mg0.05Te)10Sb2Te3 sample.