First-principles study on the lattice thermal conductivity of Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayers：Candidate materials for thermoelectric devices in high-temperature conditions

Abstract

Improvement of existing material properties is necessary for high-efficiency thermoelectric power conversion, but novel thermoelectric materials must also be predicted and synthesized. Here we solve the phonon Boltzmann transport equation using first-principles computations to examine the lattice thermal conductivity of Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayers. Results show that the frequencies at which larger gaps appear in the intermediate and high-frequency optical branches of In2Ge2Te3Se3 are lower than those of In2Ge2Se6. As a result, the phonon dispersion curve of In2Ge2Te3Se3 shifts downward. Since Te atoms are heavier than Se atoms, compared to In2Ge2Se6, In2Ge2Te3Se3 has a lower overall phonon group velocity. Furthermore, the tight coupling of the in-plane acoustic modes and the soft bending of In2Ge2Te3Se3 in the finite layer thickness coupling result in an increase in the phonon-phonon scattering, a reduction in the phonon relaxation time, and a larger Grüneisen parameter, indicating that In2Ge2Te3Se3 is more anharmonic. At a temperature of 1000 K, the sum of all these parameters results in a minimum lattice thermal conductivity of 0.05 W/mK for In2Ge2Te3Se3 and 0.24 W/mK for In2Ge2Se6. This study sheds light on the Janus In2Ge2Te3Se3 and In2Ge2Se6 bilayer thermal transport capabilities, it may open the door for achieving thermal conductivity control in applications such as thermal management and thermoelectric devices.