Decoupling thermal and electrical transport in α-MgAgSb with synergic pressure and doping strategy

Abstract

The Nowotny-Juza α-MgAgSb has been demonstrated to be a promising candidate for room-temperature thermoelectric material, owing to its ultralow lattice thermal conductivity. The challenge of enhancing its figure of merit (ZT) for commercial applications is how to effectively decouple the electrical and thermal transport with available experimental strategies. With a synergic pressure and doping strategy, we demonstrate from first principles that the bandgap of α-MgAgSb enlarges and its electrical and thermal transport can be decoupled. From the perspective of lattice dynamics, the locally vibrating three-centered Mg-Ag-Sb bonds generate multiple low-lying optical phonons which contribute large scattering channels among heat-carrying phonons and thus result in a strong anharmonicity. Under hydrostatic pressure from ambient to 50 GPa, the chemical bonds are strengthened and low-lying optical phonons move upward, which reduces the anharmonic three-phonon scattering events and thus increases lattice thermal conductivity. Under hydrostatic pressure, α-MgAgSb maintains high mechanical stability even at 550 K and 50 GPa, as verified by first-principles molecular dynamics simulations. By combining the pressure and the doping strategy to engineer density of states near the Fermi level, the thermoelectric power factor can be tuned to be significantly high while the thermal conductivity remains reasonably low. The physical insights gained from this work pave the way for decoupling electrical and thermal transport of α-MgAgSb via the synergic pressure and doping strategy toward improving its thermoelectric performance.