Ideal Strength and Deformation Mechanism in High-Efficiency Thermoelectric SnSe

The widespread use of thermoelectric conversion technology requires thermoelectric materials of high thermoelectric efficiency and high fracture strength. Single crystal SnSe shows an extremely high zT value in the moderate temperature range, but its mechanical properties have rarely been studied so far. Here we use density functional theory to determine the ideal strength and deformation mechanism of perfect SnSe single crystals for shear deformations. The lowest ideal strength of SnSe is found to be 0.59 GPa under the (100)/⟨001⟩ shear load, which agrees with experimental evidence that single crystals cleave along the (100) slip plane. The van der Waals-like Se-Sn bond, which couples the different Se-Sn layered substructures, is much softer than the covalent Se-Sn bond which constructs the Se-Sn layered substructure. This creates pathways of easy slip between Se-Sn layered substructures, which can release shear stress and lead to structural failure. Meanwhile, the layered substructures themselves can resist shearing within the (100)/⟨001⟩ slip system. These results provide a plausible atomic explanation for understanding the intrinsic mechanics of SnSe.