Anomalous vibrational dynamics in the Mg₂Zn₁₁ phase

We present a combined experimental and theoretical study of the structure and the lattice dynamics in the complex metallic alloy Mg₂Zn ₁₁, by means of neutron and x-ray scattering, as well as ab initio and empirical potential calculations. Mg₂Zn₁₁ can be seen as an intermediate step in structural complexity between the simple Laves-phase MgZn₂ on one side, and the complex 1/1 approximants and quasicrystals ZnMgAl and Zn(Mg)Sc on the other. The structure can be described as a cubic packing of a triacontahedron whose center is partially occupied by a Zn atom. This partially occupied site turned out to play a major role in understanding the lattice dynamics. Data from inelastic neutron scattering evidence a Van Hove singularity in the vibrational spectrum of Mg₂Zn₁₁ for an energy as low as 4.5 meV, which is a unique feature for a nearly-close-packed metallic alloy. This corresponds to a gap opening at the Brillouin zone boundary and an interaction between a low-lying optical branch and an acoustic one, as could be deduced from the dispersion relation measured by inelastic x-ray scattering. Second, the measured phonon density of states exhibits many maxima, indicating strong mode interactions across the whole energy range. The origin of the low-energy modes in Mg₂Zn₁₁ and other features of the vibrational spectra are studied, using both ab initio and empirical potential calculations. A detailed analysis of vibrational eigenmodes is presented, linking features in the vibrational spectrum to atomic motions within structural building blocks.