We employ first principles total energy and phonon calculations to address the structure and stability of Al2Fe. 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 MoSi2. This MoSi2 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 Al5Fe2 poses a similar problem of unexplained stability.
@article{arxiv.1107.0333,
title = {Structure and stability of Al$_2$Fe},
author = {M. Mihalkovič and M. Widom},
journal= {arXiv preprint arXiv:1107.0333},
year = {2011}
}