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Phonons et Phasons dans les quasicristaux de symetrie icosaedrique et dans leurs approximants 1/1 periodiques

Abstract : The static and dynamical responses associated with phonons and phasons are compared
in icosahedral quasicrystals and their cubic 1/1 approximants. Quasicristals and approximants share
a same local order within large atomic clusters of icosahedral shape (diameter ~ 1.6 nm). However,
the long range order is quasiperiodic in the quasicrystal whereas it is periodic in the approximant.
Quasiperiodicity yields new hydrodynamic modes called phasons. Phonons exist in the quasicrystal
and in the approximant.
We measured the distribution of the diffuse intensity near Bragg peaks in the Zn-Sc
approximant phase and in the i-Zn-X-Sc quasicristalline phases with X = Mg, Ag, Co. In the Zn-Sc
approximant, the thermal diffuse scattering due to phonons is observed. In the Zn-X-Sc phases,
there is an additional diffuse intensity signal which one is due to phasons. The extracted phason
elastic contants are shown to vary with the chemical nature of the element X.
We studied the dynamics of long-wavelength phasons using coherent X-rays in the i-Al-Pd-
Mn phase. Phasons are activated above 600°C. The dynamics is diffusive. At 650°C, for phasons
having a wavelength of 90 nm, the typical relaxation time is of the order of 100 s.
We investigated the phonon dynamics in the i-Zn-Mg-Sc and 1/1 Zn-Sc phases, and in the
i-Cd-Yb and 1/1 Cd-Yb phases using neutron and X-ray inelastic scattering. The dynamical
structure factor S(Q, E) are similar in the quasicrystal and its approximant which shows the
importance of the local order on the dynamical response of vibrational modes in those structures.
Some differences are nevertheless observed.
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Contributor : Sonia Francoual <>
Submitted on : Saturday, April 21, 2007 - 7:55:03 PM
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  • HAL Id : tel-00142770, version 1




Sonia Francoual. Phonons et Phasons dans les quasicristaux de symetrie icosaedrique et dans leurs approximants 1/1 periodiques. Physique [physics]. Université Joseph-Fourier - Grenoble I, 2006. Français. ⟨tel-00142770⟩



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