Simulated structure and thermodynamics of decagonal Al-Co-Cu quasicrystals

TL;DR

This study combines empirical potentials, molecular dynamics, Monte Carlo methods, and density functional theory to investigate the structure and thermodynamic stability of decagonal Al-Co-Cu quasicrystals, revealing entropy-driven stabilization at high temperatures.

Contribution

It introduces a comprehensive multi-method approach to model and analyze the atomic structure and stability of Al-Co-Cu quasicrystals, emphasizing the role of entropy in their stabilization.

Findings

Quasicrystals are entropically stabilized at 600-800K.

They are unstable at low temperatures, decomposing into crystal phases.

Structural differences depend on Co-rich or Cu-rich compositions.

Abstract

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.