| Issue |
J. Phys. Colloques
Volume 47, Number C3, Juillet 1986
International Workshop on Aperiodic Crystals
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|---|---|---|
| Page(s) | C3-371 - C3-377 | |
| DOI | https://doi.org/10.1051/jphyscol:1986338 | |
J. Phys. Colloques 47 (1986) C3-371-C3-377
DOI: 10.1051/jphyscol:1986338
THE STRUCTURE OF ICOSAHEDRAL MnAlSi AND MnAl
E.A. STERN1, Y. MA1, K. BAUER1 et C.E. BOULDIN21 Dept. of Physics, FM-15, University of Washington, Seattle, WA 98195, U.S.A.
2 National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.
Abstract
Extended x-ray absorption fine structure measurements show that in icosahedral (1) (MnAlSi) and (MnAl) there is a structural unit consisting of a cage of Mn atoms on the vertices of an icosahedron with an average distance between Mn atoms of 5 Å. The icosahedra are connected with an average of 3.4 ± 1.0 neighbouring icosahedra. It is suggested that the i-phase grows by randomly nucleating together Mn icosahedra along their 20 threefold directions as allowed by steric constraints, but maintaining orientation. The diffraction pattern of a computer simulation of such a model, called a randomly connected icosahedra (RCI) model, is calculated and compared with experiment. Excellent agreement occurs in the peaks when the center-to-center distance between icosahedra is 10.96 Å for i (MnAl). The diffraction widths for the RCI model are finite and vary from peak to peak, indicating an essential difference from models obtained by projection from higher dimensional space. The Hendricks-Teller model calculation is found to be a reasonable approximation to the RCI diffraction pattern. It is argued that the RCI model of a quasicrystal is favoured over ones which project from higher dimension.