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The electronic structure and thermoelectric (TE) properties of Mg2GexSn1 x (x =0.25,0.50,0.75) solid solutions are investigated by first-principles calculations and semi-classical Boltzmann theory.The special quasi-random structure (SQS) is used to model the solid solutions,which can produce reasonable band gaps with respect to experimental results.The n-type solid solutions have an excellent thermoelectric performance with maximum zT values exceeding 2.0,where the combination of low lattice thermal conductivity and high power factor (PF) plays an important role.These values are higher than those of pure Mg2Sn and Mg2Ge.The p-type solid solutions are inferior to the n-type ones,mainly due to the much lower PF.The maximum zT value of 0.62 is predicted for p-type Mg2Ge0.25Sn0.75 at 800 K.The results suggest that the n-type Mg2GexSn1-x solid solutions are promising mid-temperature TE materials.