  Atomic structures of Al-Co-Cu decagonal quasicrystals (QCs) are investigated
using empirical oscillating pair potentials (EOPP) in molecular dynamic (MD)
simulations that we enhance by Monte Carlo (MC) swapping of chemical species
and replica exchange. Predicted structures exhibit planar decagonal tilin g
patterns and are periodic along the perpendicular direction. We then
recalculate the energies of promising structures using first-principles density
functional theory (DFT), along with energies of competing phases. We find that
our $\tau$-inflated sequence of QC approximants are energetically unstable a t
low temperature by at least 3 meV/atom. Extending our study to finite
temperatures by calculating harmonic vibrational entropy, as well as anharmonic
contributions that include chemical species swaps and tile flips, our results
suggest that the quasicrystal phase is entropically stabilized at temperatur es
in the range 600-800K and above. It decomposes into ordinary (though complex)
crystal phases at low temperatures, including a partially disordered B2-type
phase. We discuss the influence of density and composition on QC phase
stability; we compare the structural differences between Co-rich and Cu-rich
quasicrystals; and we analyze the role of entropy in stabilizing the
quasicrystal, concluding with a discussion of the possible existence of "high
entropy" quasicrystals.
