Structural failure of layered thermoelectric In₄Se_3-δ semiconductors is dominated by shear slippage

In₄Se_3-δ semiconductors exhibit high zT as an n-type TE material, making them promising materials for thermoelectric (TE) applications. However, their commercial applications have been limited by the degradation of their mechanical properties upon cyclic thermal loading, making it important to understand their stress response under external loadings. Thus we applied molecular dynamics (MD) simulations using a density functional theory (DFT) derived force field to investigate the stress response and failure mechanism of In₄Se_3-δ under shear loading as a function of strain rates and temperatures. We considered the most plausible slip system (001)/<100> based on the calculations. We find that shear slippage among In/Se layered structures dominates the shear failure of In₄Se_3-δ. Particularly, Se vacancies promote disorder of the In atoms in the shear band, which accelerates the shear failure. With increasing temperature, the critical failure strength of In₄Se₃ and the fracture strain of In₄Se₃ decrease gradually. In contrast, the fracture strain of In₄Se_2.75 is improved although the ultimate strength decreases as temperature increases, suggesting that the Se vacancies enhance the ductility at high temperature. In addition, the ultimate strength and the fracture strain for In₄Se_2.75 increase slightly with the strain rate. This strain rate effect is more significant at low temperature for In₄Se_2.75 because of the Se vacancies. These findings provide new perspectives of intrinsic failure of In₄Se_3-δ and theory basis for developing robust In₄Se_3-δ TE devices.