Comparative study on the strain-dependent mechanical and electronic properties of Nb₃Al and Nb₃Sn

Nb₃Al and Nb₃Sn are important A15 compounds due to their good superconducting properties in high magnetic field. The strain coming from multiple sources would result in the degradation of superconducting performance for both of them. Extensive experimental studies have found that Nb₃Sn has higher strain sensitivity than Nb₃Al. However, most of the previous theoretical works focus on the effect of compressive hydrostatic pressure, which doesn't reflect the actual working condition. In this work, first-principles calculations were done to investigate the mechanical behaviors of the two materials under the three crystallographic uniaxial tensile loadings. Determined from the stress-strain relations, the theoretical strength of Nb₃Al is found to be slightly higher than that of Nb₃Sn, which is validated by the energy-strain curve and the corresponding phonon instability. Electronic analyses show that the property difference of the two materials is attributed to their different bonding states and strain sensitivities. The present work unveils why the superconductivity of Nb₃Sn is more sensitive to strain than Nb₃Al on the microscopic level. (paper)