The Ideal Strengths of Superconducting MgCNi₃ and CdCNi₃

The stress-strain curves under tensile deformation in the 〈100〉, 〈110〉, and 〈111〉 directions and under shear deformation in the (001)〈110〉, $\left(110\right)⟨\overline{1}10⟩$, $\left(111\right)⟨1\overline{1}0⟩$, and $\left(111\right)⟨11\overline{2}⟩$ slip systems have been systematically calculated by first-principles method to study the ideal strengths of superconducting MgCNi₃ and CdCNi₃. The ideal strengths in the three tensile directions are found to be reduced in the order of 〈100〉 → 〈110〉 → 〈111〉 and those for the four shear slip systems in the order of $\left(110\right)⟨\overline{1}10⟩\to \left(111\right)⟨11\overline{2}⟩\to \left(111\right)⟨1\overline{1}0⟩\to \left(001\right)⟨110⟩$ for both superconductors. Their lowest ideal tensile strengths are found to be larger than the corresponding highest ideal shear strengths, which could explain why both superconductors have the ductility. The obtained lattice constants and elastic properties coincide well with the the available experimental and theoretical values.