Deformation Mechanisms of Clathrate tI-Na₂ZnSn₅

Thermoelectric semiconductor tI-Na₂ZnSn₅ has an impressive cage-like framework structure with Na helical tunnels along the [100] and [010] directions. This Na ion channel enables the material to obtain extremely low thermal conductivity, while the intrinsic mechanical properties of tI-Na₂ZnSn₅ have not been explored. Here, we report the ideal strength as well as the deformation mechanisms of the compound under shear/tension using density functional theory. tI-Na₂ZnSn₅ has the lowest shear strength of 2.01 GPa along the (001)/[110] slip system. During the shear deformation, the Zn-Sn bonds are gradually stretched, softening the Zn-Sn cage-like framework, which induces the rearrangement of the Na ions inside the cage. This can maintain the structural integrity. The failure mechanism is attributed to the continuous softening of the Zn-Sn bond and the breaking of the covalent Sn-Sn bond, which leads to the deconstruction of the Zn-Sn covalent framework. The tensile strength along the [001] direction is significantly lower than that along the [010] direction, owing to the stretching of Sn-Sn bonds, which serve as the “pillars” of the helical tunnels. This work provides new insights into understanding the failure mechanisms of clathrates with ion channel structures, which will facilitate the design of robust ion channel clathrates.