It is shown that the extended softening in $k$ space of the [111] transverse acoustic phonon in ${\mathrm{Nb}}_3$Sn seen recently by neutron measurements is in agreement with the predictions of a phenomenological Landau description of the structural phase transition in the $A-15\mathrm{}$ compounds. This indicates the feasibility of a calculation of the partial phonon density of states of modes directly coupled to the order parameter driving the structural transition, as a function of temperature.

• Received 22 November 1978

DOI:https://doi.org/10.1103/PhysRevB.19.4023