Local Order in Single Grain Cd-Yb Icosahedral Phase
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Local Order in Single Grain Cd-Yb Icosahedral Phase
Y. Q. Wu, M. J. Kramer and T. A. Lograsso Metals and Ceramics Sciences, Ames Laboratory, Ames, Iowa 50011 ABSTRACT Single grains of icosahedral Cd84Yb16 quasicrystalline phase were characterized using high resolution transmission electron microcopy (HRTEM) technique. Image reconstruction of the obtained multiple through-focal series for both the two and five-fold axes are consistent with the local cluster structure of concentric polyhedra consisting of Cd tetrahedron (1/3 occupied icosahedron), Cd dodecahedron, Yb icosahedron and a Cd icosidodecahedron. Edge-sharing of the clusters can result in complete tiling of the 2-D projections. Simple matching rules are being investigated to construct a 3-D model. INTRODUCTION Understanding the unique structure of QC and what gives rise to the aperiodic structure has been an open question since quasicrystals were first discovered by Dan Shechtman in 19841. Whether the rare-earth quasicrystals are based upon any cluster model has been openly debated and remains uncertain. Recently, M. J. Kramer et al. studied the local atomic structure of R-Mg-Zn ternary QC and identified the atomic positions2, showing that the face centered icosahedral quasicrystals are based upon Bergman-like clusters and the rare-earth provides a unique probe to determine the atomic structure3. However, the occupants of those sites have not been uniquely determined. All stable quasicrystals (QC) consist of at least three elements before the first binary icosahedral cadmium-ytterbium (Cd5.7Yb) quasicrystalline reported in 2000 by Tsai et al.4 and Guo et al.5. Stable binary phases (aperiodic and related crystalline) provide an intriguing opportunity to gain better insight into their structural versus chemical stabilities, which has been elusive in ternary and higher order systems. The complexity of performing site occupation determinations in a binary compound is considerably reduced compared to the ternary. Therefore, a binary system will be the best candidate to study the basis for the construction of crystallographic model of quasiperiodic structure. The general simplicity of a binary system presents numerous advantages in both computational and experimental investigations of structure. In this paper, we will try to investigate the local atomic structure of a binary icosahedral Cd84Yb16 quasicrystal6 using high resolution transmission electron microcopy (HRTEM) and simulation techniques, in an attempt to provide an insight into the role of short range order in controlling the stability of quasicrystalline compounds. EXPERIMENTAL DETAILS Bulk single grain of icosahedral Cd84Yb16 quasicrystalline was prepared using the
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Bridgman method. HRTEM foils were firstly electron discharge machined from larger grains on the order of 0.75 cm3 and then mechanically thinned to ~200 µm. The samples were cut to circular plates of 3 mm in diameter and 300 µm in thickness and were electrolyte-polished to make final HRTEM specimens. Special care had to be
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