Electron-Phonon Interaction and Phonon Softening in Systems with Magnetovolume Instabilities

The interrelation of Invar and martensitic behaviour in transition metal alloys like Fe65Ni35 is discussed on the basis of ab initio calculations for stoichiometric systems like Fe3Ni. We also examined face centered cubic iron as a model system for Antiinvar which like Invar shows a pronounced phonon softening. Neutron scattering experiments on Invar systems have shown that the TA1 phonon mode softens for small wavevectors q' along the [l101 direction in the fcc lattice. To day there exist only a few ab initio calculations for martensitic transformations in nonmagnetic compounds. We have extended these kinds of investigations to the magnetic case. Our numerical calculations are based on the first principles theory of Varma and Weber who considered the electron-phonon interaction in second order perturbation theory. We have calculated renormalized phonon dispersion curves with the help of an appropriate set of tight-binding states fitted to ab initio band structure results. Our numerical results reproduce the experimental findings. In particular we observe that phonon softening in systems with magnetovolume instabilities is caused by the long-range contribution to the electronphonon interaction.