X-RAY DIFFRACTION STUDIES ON DIFFUSE SCATTERING IN SINGLE QUASICRYSTALS. F. Dénoyer, Laboratoire de Physique des Solides, Associé au CNRS, Bâtiment 510, Université Paris-Sud, 91405 Orsay Cédex, France.

In addition to the diffraction peaks observed in icosahedral and decagonal phases, X-ray diffraction patterns reveal diffuse scattering which also exhibits icosahedral (F. Dénoyer, G. Heger, M. Lambert, J.M. Lang, P. Sainfort, J. Physique 48 (1987) 1357-1361, P. Donnadieu, F. Dénoyer submitted) or decagonal symmetry (F. Frey, W. Steurer, J. of Non-Cryst. Solids, 153&154 (1993) 600-605, M. Fettweis, P. Launois, F. Dénoyer, R. Reich, M. Lambert, Phys. Rev. B49 (1994) 15573-15587, M. Fettweis, P. Launois, R. Reich, R. Wittmann, F. Dénoyer, Phys. Rev. B51 (1995) 6700-6703). The origin of this diffuse scattering is of interest since it may allow to understand the mechanism of formation of quasicrystals.

Spherical shells (or arcs) of diffuse scattering in electron and X-ray diffraction patterns seem to be a characteristic of icosahedral phases. These diffuse features have been interpreted as the result of an intrinsic packing disorder of the alloy structures, most of the models being based on dense randomly packing distributions of icosahedral clusters of atoms (A.I. Goldman, C.A. Guryan, P.W. Stephens, J.M. Parsey, Jr., G. Aeppli, H.S. Chen, F.W. Gayle, Phys. Rev. Lett. 61 (1988) 1962-1965, J.L. Robertson, S.C. Moss, J. of Non-Cryst. Solids 153&154 (1993) 77-82, P.C. Gibbons, K.F. Kelton, J. of Non-Cryst. Solids 153&154 (1993) 165-171, M.E.J. Newman, C.L. Henley, M. Oxborrow, Phil. Mag. B71 (1995) 991-1013).

In decagonal alloys (F. Frey, W. Steurer, J. of Non-Cryst. Solids, 153&154 (1993) 600-605, M. Fettweis, P. Launois, R. Reich, R. Wittmann, F. Dénoyer, Phys. Rev. B51 (1995) 6700-6703, M. Fettweis, PhD thesis Université ParisXI, France (1994)), localized diffuse scattering disappears above 800deg.C. At room temperature, their shapes depend on the chemical composition of the alloy and are quite different according as the sample is in a decagonal metastable quenched quasicrystalline state or in a stable decagonal-like microcrystalline state.