Atomic dynamics in Al-rich Al-Co alloys near the composition of the decagonal quasicrystal

We used realistic Al-Co pair potentials [R. Phillips, J. Zou, A. E. Carlsson, and M. Widom, Phys. Rev. B 49, 9322 (1994); J. A. Moriarity and M. Widom, ibid. 56, 7905 (1997)] to study the atomic dynamics of Al_1-xTMₓ crystalline structures with the fractional content x of the transition metal (TM) atom x0.3. Our list comprises rather simple structures of Al₃Ni and Al₅Co₂ alloys, complex structures related to the decagonal quasicrystal (Al₉Co₂Ni, O-Al₁₃Co₄), and a model of the crystalline approximant of the decagonal quasicrystal d-AlNiCo. Within the harmonic approximation, we assess the impact of the structural complexity on the phonon density of states, sound velocity, Debye-Waller factor, and the character of the phonon states at low energies. In complex structures related to the decagonal quasicrystal, a significant fraction of low-energy vibrations have nonacoustic, strongly localized character. In a molecular-dynamics annealing of the decagonal approximant model performed at elevated temperature, a fraction of aluminum atoms display signs of diffusive motion, while the equilibrium positions of the cobalt atoms do not change.