Higher thermoelectric performance of Zintl phases (Eu0.5Yb0.5)1-xCaxMg2Bi2 by band engineering and strain fluctuation

Complex Zintl phases, especially antimony (Sb)-based YbZn0.4Cd1.6Sb2 with figure-of-merit (ZT) of similar to 1.2 at 700 K, are good candidates as thermoelectric materials because of their intrinsic electron-crystal, phonon-glass nature. Here, we report the rarely studied p-type bismuth (Bi)-based Zintl phases (Ca,Yb,Eu)Mg2Bi2 with a record thermoelectric performance. Phase-pure EuMg2Bi2 is successfully prepared with suppressed bipolar effect to reach ZT similar to 1. Further partial substitution of Eu by Ca and Yb enhanced ZT to similar to 1.3 for Eu0.2Yb0.2Ca0.6Mg2Bi2 at 873 K. Density-functional theory (DFT) simulation indicates the alloying has no effect on the valence band, but does affect the conduction band. Such band engineering results in good p-type thermoelectric properties with high carrier mobility. Using transmission electron microscopy, various types of strains are observed and are believed to be due to atomic mass and size fluctuations. Point defects, strain, dislocations, and nanostructures jointly contribute to phonon scattering, confirmed by the semi-classical theoretical calculations based on a modified Debye-Callaway model of lattice thermal conductivity. This work indicates Bi-based (Ca,Yb,Eu)Mg2Bi2 is better than the Sb-based Zintl phases.