Skutterudites based on CoSb_3 have high thermoelectric efficiency, but the low fracture strength is a serious consideration for commercial applications. To understand the origin of the brittleness in CoSb_3, we examine the response along various shear and tensile deformations using density functional theory. We find that the Co–Sb bond dominates the ideal strength. Among all the shear and tensile deformation paths, shearing along the (001)/⟨100⟩ slip system has the lowest ideal strength, indicating it is the most likely slip system to be activated under pressure. We also find that, because the Sb–Sb covalent bond is softer than the Co–Sb bond, the Sb-rings are less rigid than the Co–Sb frameworks, which leads to the Sb-rings softening before the Co–Sb frameworks. Further deformation leads to deconstruction of Sb-rings and collapse of Co–Sb frameworks, resulting in structural failure. Moreover, we find that filling of the CoSb_3 void spaces with such typical fillers as Na, Ba, or Yb has little effect on the ideal strength and failure mode, which can be understood because they have little effect on the Sb-rings.
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