Stacking-dependent thermoelectric transport in layered Sc_2Si_2Te_6 from first principles

TL;DR

This study uses first-principles calculations to explore how different stacking sequences in layered Sc_2Si_2Te_6 affect its thermoelectric properties, revealing stacking-dependent electronic and thermal behaviors.

Contribution

It systematically analyzes the impact of stacking polymorphism on thermoelectric performance, highlighting the significance of stacking control for optimizing thermoelectric efficiency.

Findings

ABC stacking yields the highest ZT due to low thermal conductivity.

Stacking sequence significantly alters electronic band degeneracies.

Four-phonon scattering further reduces lattice thermal conductivity in ABC stacking.

Abstract

Stacking polymorphism is a common characteristic of van der Waals layered materials and can substantially modify their physical properties. Here, based on first-principles calculations combined with electron and phonon transport theories, we systematically investigate the thermodynamic stability, electronic structure, lattice dynamics, and thermoelectric performance of Sc_2Si_2Te_6 with three high-symmetry stacking sequences, namely, AA, AB, and ABC. We find that the AA- and AB-stacked structures are nearly degenerate in energy with the experimentally reported ABC phase, and that the maximum sliding barrier among these stacking sequences is only about 10~meV/atom, thereby accounting for the stacking faults observed experimentally. These three stacking sequences exhibit distinct electronic structures, with the conduction-band minimum being highly sensitive to the stacking sequence. As a consequence, the conduction-band degeneracies are 12, 2, and 8 for the ABC, AA, and AB stackings, respectively, leading to markedly different electronic transport properties near the band edge. The lattice thermal conductivity is governed primarily by three-phonon scattering, whereas four-phonon scattering provides an additional reduction, particularly in the ABC stacking. Among the three structures, the AB stacking exhibits the lowest lattice thermal conductivity owing to its stronger three-phonon scattering and lower phonon group velocity. As a result, the maximum thermoelectric figure of merit, ZT, is achieved in the ABC structure, followed closely by the AB structure, whereas the AA structure shows a substantially reduced value. These results demonstrate that the stacking sequence exerts a non-negligible influence on the thermoelectric performance of Sc_2Si_2Te_6 and suggest that suppressing the formation of the AA stacking is important for achieving high thermoelectric performance.