Role of vibrational entropy in the stabilization of the high-temperature phases of iron

TL;DR

This study investigates how vibrational entropy influences the stabilization of high-temperature phases of iron, revealing that vibrational entropy mainly stabilizes the bcc phase while electronic entropy contributes equally to the fcc phase.

Contribution

It provides detailed phonon dispersion analysis near phase transitions and quantifies the roles of vibrational and electronic entropy in phase stabilization.

Findings

High-temperature bcc phase stabilized mainly by vibrational entropy.

Fcc phase stabilization involves significant electronic entropy contribution.

Phonon softening correlates with phase transitions.

Abstract

The phonon dispersions of the bcc and fcc phases of pure iron ({\alpha}-Fe, {\gamma}-Fe and {\delta}-Fe) at ambient pressure were investigated close to the respective phase transition temperatures. In the open bcc structure the transverse phonons along T1 [{\xi}{\xi}0] and T1 [{\xi}{\xi}2{\xi}] are of particularly low energy. The eigenvectors of these phonons correspond to displacements needed for the transformation to the fcc {\gamma}-phase. Especially these phonons, but also all other phonons soften considerably with increasing temperature. Comparing thermodynamic properties of the fcc and the two bcc phases it is shown that the high temperature bcc phase is stabilized predominantly by vibrational entropy, whereas for the stabilization of the fcc phase electronic entropy provides an equal contribution.