Theory of Structural Phase Transition inNb3Sn

Abstract

We adopt the constant-density-of-states one-dimensional model for the $d$ bands but carefully include the effects of the sublattice displacements on the lattice energy and on the $d$ electrons. Then, we are able to explain quantitatively the behavior of various properties of ${\mathrm{Nb}}_3$Sn in both the cubic and tetragonal phases such as the shear modulus, the spontaneous tetragonal strain, and sublattice shift, with a reasonable set of parameters (e.g., density of states 3.2 states/eV atom, Fermi temperature 195 K). We deduce the energy of the ${\Gamma }_{12}$ optical phonon to be 21 meV.