Invar, AF-Invar, Anti-Invar and Martensite in FCC Fe-Based Alloys : An Attempt to Order this Chaos

Abstract
Systematics concerning magnetic and structural phase transitions within Fe-based sytems can be achieved if experimental data are analyzed as a function of composition and the atomic volume. The findings can be understood from calculations of the exchange coupling of pure fcc Fe as a function of the n.n. distance. Decisive is that the volume increase going along with Invar, AF-Invar, Anti-Invar as well as martensite transitions originates from a charge transfer between antibonding electronic orbitals with t_2g symmetry and AF exchange to non-bonding orbitals with e_g symmetry and FM exchange. Which of the effects is leading at a given composition is a question of the position of the Fermi energy relative to these electronic levels. The question of how an effect manifests itself as a function of temperature is a question of the different strength of the t_2g- and e_g-electron-phonon coupling.