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The layered In 4 Se 3 based material is recognized as a state-of-the-art n-type thermoelectric material for the middle temperature range of 500 to 900 K. Despite excellent thermoelectric properties, its inferior mechanical properties restrict its commercial possibilities. In this work, we use quantum mechanics (density functional theory) to investigate the ideal strength and failure mechanisms of ideal and Se deficient In 4 Se 3 under pure shear and biaxial shear loads. We found that the lowest ideal shear strength of ideal In 4 Se 3 is 1.25 GPa along the (100)/<001> slip system. Slippage between the In/Se layer dominates its deformation and failure. With Se vacancies, the ideal strength of In 4 Se 2.75 drops to 1.00 GPa, while the failure mechanism remains almost the same as that of ideal In 4 Se 3 . Moreover, under biaxial shear loads (as in nanoindentation experiments), the ideal strength of In 4 Se 3 increases to 1.50 GPa, with compression now accounting for the failure. Even so, In 4 Se 3 shows poorer mechanical properties under biaxial shear loads. These insights into the deformation and failure mechanism of In 4 Se 3 compounds should help suggest designing modifications to improve mechanical properties.

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