Structural instability and electronic excitations in Nb3Sn

We have performed an extensive first-principles study of the cubic-to-tetragonal structural phase transformation in the A15 compound Nb3Sn, using optimized norm-conserving pseudopotentials and the plane-wave basis set. Our calculations confirm that cubic Nb3Sn is unstable with respect to the sublattice distortions of Gamma(12) symmetry discovered by Shirane and Axe [Phys. Rev. B 4, 2957 (1971)]. However, the cubic phase is stable with respect to the tetragonal strain if the atoms are frozen in their ideal positions. The structural instability with respect to the sublattice distortions is explained by an unusually strong electron-phonon coupling between the Gamma(12) phonons and the states at the Fermi level, while no such anomalous coupling has been found for the tetragonal strain. Finite-temperature electronic-structure effects are modeled using the exact Fermi-Dirac distribution function in self-consistent local-density-approximation calculations. Electronic excitations are found to stabilize the cubic phase above T-M = 450 K. The model correctly reproduces several experimental findings, including the softening of the elastic constant C' and zone-center optical Gamma(12) frequency nu(Gamma 12) as T-->T-M, as well as their subsequent stiffening below T-M. Furthermore, C' is found to approach zero at T-M, while nu(Gamma 12) does not, which is again consistent with the observed behavior of Nb3Sn.