Ground-state crystal structures of superconducting Nb3Al and the phase transformation under high pressures

Niobium aluminide Nb3Al, a typical A15 conventional superconductor, has been believed to adopt the cubic β-W type structure with a space group of Pm3¯n at ambient pressure since its discovery. Herein, we report a new crystal structure with a space group of C2/c, as predicted from the ab initio evolutionary algorithm, is energetically more favorable than the A15 phase at ambient pressure and low temperature. Phonon calculations indicate this phase is dynamically stable. The application of the Allen-Dynes modified McMillan equation to the C2/c phase yields a superconducting transition temperature Tc in the range of 17.44 – 19.48 K, which is in good agreement with experiments (18.8 K). A key difference between the A15 and C2/c crystal structures is the distortion of the Nb atomic chains, which are believed to be closely related to the superconducting behavior. Based on ab initio molecular dynamics simulations, we find that Nb3Al transforms from the newly discovered C2/c phase to the standard A15 phase as temperature increases. Furthermore, another new phase with a space group of Cmcm is found to become stable as a hydrostatic pressure is applied.