First principles study of the structure and properties of Nb-Sn alloys under high pressure

We conducted a systematic high-pressure study on the Nb-Sn system using first-principles calculations based on density functional theory. The USPEX software package was used to investigate the crystal structure of Nb-Sn alloys under pressures ranging from 0 to 30 GPa. As a result, five new ground-state structures for the Nb-Sn alloys were predicted: Pm-3_Nb7Sn, I4/mmm_Nb2Sn, P6(4)22_NbSn2, P4/nmm_NbSn3 and Cmcm_NbSn3. According to our calculated results, it is found that eight compounds are thermodynamically and dynamically stable in Nb-Sn system under different pressures. These compounds are: Pm-3_Nb7Sn, Pm 3n_Nb3Sn(A15), Fddd_NbSn2, I4/mmm_Nb2Sn, P6(2)22_NbSn2, P6(4)22_NbSn2, P4/nmm_NbSn3, and Cmcm_NbSn3. The calculations of electronic properties indicate that structures exhibit metallic behavior. In addition, the superconducting transition temperatures of Pm-3_Nb7Sn at atmospheric pressure is similar to 5.15 K, and our results also suggest that the superconducting properties arises mainly from the coupling between Nb-4d electrons and phonons near the Fermi level.