Strong Polar Optical Phonon Screening and Softening Enhance the Thermoelectric Performance of Zintl Compounds

Abstract

Ternary CaAl₂Si₂-structure-type Zintl compounds are promising p-type counterparts to n-type Mg₃(Sb, Bi)₂ for thermoelectric energy conversion. However, many of these p-type Zintl compounds suffer from low carrier concentration and mobility, resulting in poor thermoelectric performance. Here, it is revealed that their ultralow mobility stems from strong polar optical phonon scattering, and demonstrate that their electrical transport properties can be dramatically boosted by employing a screening effect. By employing isovalent alloying with Cd and Yb, along with Li aliovalent acceptor doping in CaMg₂Sb₂ to increase carrier concentration and induce a strong screening effect, a significant improvement in carrier mobility and, consequently, the power factor is achieved. Moreover, isovalent alloying weakens chemical bonding, causing the softening and deceleration of both acoustic and optical phonons and, thus, a reduction in lattice thermal conductivity. As a result, a ZT of 1.1 is achieved in the Ca_0.69Yb_0.3Li_0.01Mg_1.5Cd_0.5Sb₂ sample at 773 K, representing a 30-fold increase compared to the pristine CaMg₂Sb₂. It is also proposed that the polar coupling constant can serve as a criterion for identifying materials with low intrinsic carrier concentration and mobility but with potential for thermoelectric applications facilitating the development of other thermoelectric materials beyond CaAl₂Si₂-structure-type Zintl compounds.