Chemical Pressure Induced Efficient Carrier Concentration Optimization for High Thermoelectric Performance in SmS

Carrier concentration optimization is considered to be fundamentally important in thermoelectric investigation. Unlike the conventional method of aliovalent doping, this study proposes a distinctive strategy to modulate carrier concentration via introducing local chemical pressure, as demonstrated in rock-salt SmS, a promising n-type thermoelectric material with variable valence element. By considering different dopants, it is found that Y doping can provide 6-12 times more extra electrons at the same doping content as compared to Sc doping although the two elements are in same main group. Further study suggests that Y doping can induce severe local chemical pressure and result in a change in the valence state of Sm from 2⁺ to 3⁺, giving rise to many more extra electrons. Finally, the peak power factor value of Y-doped SmS reaches a record high value of ∼1.65 mW m^-1 K^-2 at 1023 K. Our findings provide unique insights for tuning the carrier concentration with high efficiency in the compounds containing a mixed valence-state element.