  Bi2Te3-xSex family has been the n-type start-of-the-art thermoelectric
materials near room temperatures (RT) for more than half-century, which
dominates the active cooling and novel waves harvesting application near RT.
However, the drawbacks of brittle nature and Te-containing restrict the further
applications exploring. Here, we show that a Mg3+{\delta}SbxBi2-x family
((ZT)avg =1.05) could be a promising substitute for the Bi2Te3-xSex family
((ZT)avg =0.9-1.0) in the temperature range of 50-250 {\deg}C based on the
comparable thermoelectric performance through a synergistic effect from the
tunable band gap using the alloy effect and the suppressible Mg-vacancy
formation using interstitial Mn dopant. The former is to shift the optimal
thermoelectric performance to near RT, and latter is helpful to partially
decouple the electrical transport and thermal transport in order to get an
optimal RT power factor. A positive temperature-dependence of band gap
suggested this family is also a superior medium-temperature thermoelectric
material for the significantly suppressed bipolar effect. Furthermore, a two
times higher mechanical toughness, compared with Bi2Te3-xSex family,
consolidates the promising substitute for the start-of-art n-type
thermoelectric materials near RT.
