Rational composition engineering toward high thermoelectric performance in p-type EuMg2Sb2-based materials

Mg-based compounds are emerging materials for efficient thermoelectric conversion, as exemplified by n-type Mg3Sb2-based materials, while a high performing p-type Mg3Sb2 material is lacking. EuMg2Sb2, a derivative of Mg3Sb2, has revealed potential as a high-performance alternative to p-type Mg3Sb2-based materials, while pristine EuMg2Sb2 suffers from low carrier concentration. Herein, we reveal that both Zn substitution for Mg and Yb substitution for Eu not only boost the carrier concentration and mobility of p-type EuMg2Sb2-based materials, but also sharply diminish the lattice thermal conductivity via enhancing phonon scattering. Band structure calculation shows that Zn substitution minimizes the energy difference between different bands at the valence band maximum and reduces the band effective mass, leading to optimized band structure. Combined with the role of Ag doping in increasing the carrier concentration and power factor, p-type Eu0.5Yb0.5Mg1.0975Zn0.9975Ag0.005Sb2 sample obtains a maximal dimensionless figure of merit (zT) value of 1.18 at 823 K, which compares favorably to those of EuMg2Sb2 and other p-type Mg-Sb-based Zintl materials. This study demonstrates the efficacy and importance of delicate composition engineering in boosting the thermoelectric performance of p-type EuMg2Sb2.