ELECTRON-PHONON INTERACTION AND PHONON SOFTENING IN SYSTEMS WITH MAGNETOVOLUME INSTABILITIES

The interrelation of Invar and martensitic behaviour in transition met al alloys like Fe65Ni35 is discussed on the basis of ab initio calcula tions for stoichiometric systems like Fe3Ni. We also examined face cen tered cubic iron as a model system for Antiinvar which like Invar show s a pronounced phonon softening. Neutron scattering experiments on Inv ar systems have shown that the TA(1) phonon mode softens for small wav evectors ($) over right arrow q along the [110] direction in the fee l attice. To day there exist only a few ab initio calculations for marte nsitic transformations in nonmagnetic compounds. We have extended thes e kinds of investigations to the magnetic case. Our numerical calculat ions are based on the first principles theory of Varma and Weber who c onsidered 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 i nitio band structure results. Our numerical results reproduce the expe rimental findings. In particular we observe that phonon softening in s ystems with magnetovolume instabilities is caused by the long-range co ntribution to the electron-phonon interaction.