Lattice Dynamics ofNb3Sn-Type Compounds

It is shown that, in the cubic phase, the doubly degenerate ${\Gamma }_{12}$ mode is the only long-wavelength optical mode which is temperature dependent in the harmonic approximation. However, it does not become unstable on cooling because it would drive a shear acoustic mode unstable first. The elastic constants are calculated by the method of long waves. Only $C_{11}-C_{12}$ is temperature dependent. Its calculated values for ${\mathrm{Nb}}_3$Sn are in fair agreement with experiment.