Hc. Herper et al., ELECTRON-PHONON INTERACTION AND PHONON SOFTENING IN SYSTEMS WITH MAGNETOVOLUME INSTABILITIES, Journal de physique. IV, 5(C2), 1995, pp. 129-134
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.