The surface structural study of an icosahedral ͑i͒ quasicrystal of the Cd-Yb family is presented. Comparison of bias-dependent scanning tunneling microscopy data from the fivefold surface of i-Ag-In-Yb with the refined bulk model of isostructural i-Cd-Yb indicates that surfaces are formed at bulk planes intersecting the center of the rhombic triacontahedral clusters, the building blocks of the Cd-Yb family quasicrystals. These observations open up the possibility of the use of this material as a template for epitaxial structures.

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We present a combined experimental and theoretical study of the structure and the lattice dynamics in the complex metallic alloy Mg 2 Zn 11 , by means of neutron and x-ray scattering, as well as ab initio and empirical potential calculations. Mg 2 Zn 11 can be seen as an intermediate step in structural complexity between the simple Laves-phase MgZn 2 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 Mg 2 Zn 11 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 Mg 2 Zn 11 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.