First-principles calculations of cohesive energies in theAl−Cobinary alloy system

Abstract

The phase diagram of the $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Co}$ binary alloy system is intensively studied because of its importance for understanding decagonal quasicrystals, but remains imprecisely known due to the occurrence of many competing complex structures with composition close to ${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}$. We apply first-principles total energy calculations to compare the cohesive energies of known and hypothetical structures. Our results confirm the experimentally established phase diagram in every detail except near ${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}$, where the reported phases (Pearson symbols mC102 and oP102, both well-known decagonal quasicrystal approximants) turn out to be unstable at low temperatures. They may be stabilized at high temperatures by the entropy of aluminum vacancy hopping and low frequency vibrational modes. Under molecular dynamics a subset of Al atoms displays nearly liquid diffusive motion.