Structure and stability of Al$_2$Fe

TL;DR

This study uses first principles calculations to investigate the structure and stability of Al$_2$Fe, revealing that the experimentally reported triclinic form is unstable compared to a simpler MoSi$_2$-type structure, with stability influenced by vibrational entropy.

Contribution

It provides a detailed computational analysis showing the instability of the reported triclinic Al$_2$Fe structure and explains its high-temperature stability through vibrational entropy effects.

Findings

The triclinic structure is unstable relative to MoSi$_2$-type structure.

High vibrational entropy stabilizes the triclinic form at high temperatures.

The stable MoSi$_2$-type structure is nonmagnetic and insulating.

Abstract

We employ first principles total energy and phonon calculations to address the structure and stability of Al$_2$Fe. This structure, which is reported as stable in the assessed Al-Fe phase diagram, is distinguished by an unusually low triclinic symmetry. The initial crystallographic structure determination additionally featured an unusual hole large enough to accommodate an additional atom. Our calculations indicate the hole must be filled, but predict the triclinic structure is unstable relative to a simpler structure based on the prototype MoSi$_2$. This MoSi$_2$ structure is interesting because it is predicted to be nonmagnetic, electrically insulating and high density, while the triclinic structure is magnetic, metallic and low density. We reconcile this seeming contradiction by demonstrating a high vibrational entropy that explains why the triclinic structure is stable at high temperatures. Finally, we note that Al$_5$Fe$_2$ poses a similar problem of unexplained stability.