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

Abstract

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 ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{TM}}_{x}$ crystalline structures with the fractional content x of the transition metal (TM) atom $x\ensuremath{\lesssim}0.3.$ Our list comprises rather simple structures of ${\mathrm{Al}}_{3}\mathrm{Ni}$ and ${\mathrm{Al}}_{5}{\mathrm{Co}}_{2}$ alloys, complex structures related to the decagonal quasicrystal $({\mathrm{Al}}_{9}{\mathrm{Co}}_{2}\mathrm{Ni},$ O-${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}),$ 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.