OBSERVING AND UNDERSTANDING ARCS OF DIFFUSE SCATTERING FROM QUASICRYSTALS. P. C. Gibbons, Department of Physics, Washington University, One Brookings Drive, St. Louis, MO 63130, USA

Localized arcs of diffuse scattering are caused by particular short-range correlations between bonded icosahedral clusters. They were observed in AlMn, the first quasicrystal discovered, after annealing. They were observed in the TiMnSi quasicrystal on the day of its discovery; in that material they appear with high contrast and their shapes and locations in reciprocal space were mapped in detail. Electron diffraction in the TEM, on selected planes in reciprocal space, produces two-dimensional images from which the full three-dimensional structure of the arcs can be inferred. X-ray diffraction has also been useful in observing the arcs, especially in materials in which their contrast is not as great as in TiMnSi. TEM investigations demonstrated that the disorder producing the arcs in TiMnSi is topological rather than chemical. Simulations of the scattering of waves from icosahedral glass models and from random canonical-cell tiling models contain arcs very similar to those observed. They can be traced to the contribution of nearest-neighbor icosahedral clusters to the pair correlation function. Improving the quality of the glasses (as assessed by the widths of the simulated diffraction peaks) with constraints that force a uniform, high density of clusters increases the contrast of the arcs in the simulations. In the best glasses weak diffraction peaks can be resolved within the arcs, like those sometimes seen in the measurements. Too-tight constraints produce a crystal structure, the 1/1 1.3 -nm cubic approximant (a bcc cell with icosahedral clusters at the corner and body center sites). Local correlations in the glass like those in the 1/1 crystal phase are strongly suggested as the source of the arcs. Modeling of these quasicrystals as canonical-cell tilings has led to the same conclusion; indeed, a highly constrained glass is structurally identical to a random canonical-cell tiling. This role of local correlations has been confirmed in a pleasingly direct way in a simulation of the diffraction from polycrystalline 1/1 structure, in which only grain orientations for which the icosahedral clusters in the bcc phase could connect coherently across grain boundaries were allowed. With grain sizes of 1.3 nm, excellent agreement with the data was obtained.