Clathrates are potential "phonon-glass, electron-crystal" thermoelectric semiconductors, whose structure of polyhedron stacks is very attractive. However, their mechanical properties have not yet met the requirements of industrial applications. Here, we report the ideal strength of element-substituted type-I and type-VIII clathrates and the shear deformation mechanism by using density functional theory. The results show that the framework element is the determinant of the intrinsic mechanical properties of the clathrates and is affected by sequential weakening of Si-Ge-Sn. The highest ideal shear strength is 8.71 GPa for I-Ba8Au6Si40 along the (110)/[001] slip system, which is attributed to the formation of higher-energy Si-Si covalent bonds. Meanwhile, the ideal shear strength of Ba-filled I/VIII clathrates (4.51/2.65 GPa) is higher than that of Sr-filled clathrates (3.64 GPa/1.91 GPa). In addition, the strength and ultimate strain of VIII-Ba8Ga16Sn30 can be significantly increased by the structural coordination accommodating with the stiffness of the Ga-Ge bond to achieve simultaneous bond breaking. Our findings demonstrate that the element substitution strategy is an effective approach for designing highly robust clathrates.
Keywords: DFT; element substitution; failure mechanisms; ideal shear strength; thermoelectric clathrate.