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H. Euchner, M. Mihalkovic, F. Gaehler, M.R. Johnson, H. Schober, S. Rols, E. Suard, A. Bosak, S. Ohhashi, A. P. Tsai, S. Lidin, C.P. Gomez, J. Custers, S. Paschen, M. de Boissieu:
"Anomalous vibrational dynamics in the Mg₂Zn₁₁ phase";
Physical Review B, 83 (2011), 14; 144202.

We present a combined experimental and theoretical study of the structure and the lattice dynamics in the complex metallic alloy Mg2Zn11, 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.

http://dx.doi.org/10.1103/PhysRevB.83.144202

http://link.aps.org/doi/10.1103/PhysRevB.83.144202