High Thermoelectric Performance of CaMg2Bi2 Enabled by Dynamic Doping and Orbital Alignment

Substantial progress in improving the thermoelectric performance of CaMg2Bi2-based materials has been made, yet existing reports barely discuss the effect of intrinsic Bi impurity on the transport properties. In this study, the first-principles calculation shows that the Bi-rich environment associated with Bi impurities facilitates the reduction of cation vacancy formation energy and then increases the carrier concentration. In addition, in the samples containing Bi impurity, a dynamic doping behavior caused by the redissolving of Bi second phase into the matrix with increasing temperature is identified experimentally, which force the carrier concentration to approach the optimal carrier concentration n(H,opt), leading to a more than a 5 times enhancement of the figure-of-merit (ZT) compared to the single-phase CaMg2Bi1.94. Moreover, by doping Ba and Yb on the Ca Site, orbital alignment is realized and phonon scattering is also enhanced. The synergistical optimization of electrical and thermal transportation brings a peak ZT of 1.24 at 873 K and a record ZT(ave) of 0.86 (300-873 K) in (Ca0.5Yb0.25Ba0.25)(0.995)Na0.005Mg2Bi1.98 sample.

Automated summary

Substantial progress has been made in improving the thermoelectric performance of CaMg2Bi2-based materials. This study reveals the effect of intrinsic Bi impurity on the transport properties and demonstrates that the Bi-rich environment associated with Bi impurities can increase the carrier concentration and enhance the figure-of-merit (ZT). Additionally, doping Ba and Yb on the Ca Site aligns the orbitals and enhances phonon scattering, leading to further optimization of electrical and thermal transportation.