Dynamics of phason diffusion in icosahedral Al-Pd-Mn quasicrystals
by Michael Feuerbacher and Daniel Caillard
Quasicrystals are materials possessing long-range order and a structural symmetry incompatible with translational symmetry. Diffraction patterns of quasicrystals exhibit sharp Bragg peaks, which, for instance in the case of icosahedral phases, show fivefold rotational symmetry. The structure of quasicrystals can be described in terms of a higher-dimensional hyperspace, which introduces additional degrees of freedom for the formation of defects. Phasons are specific defects of a quasicrystalline structure, which in a tiling description can be visualized as configuration flips leading to local matching-rule violations. Physically, these flips are realized by atomic displacements, which may involve numerous individual local atom jumps.
Levine et al., in a very general treatment, considered the hydrodynamic variables of icosahedral and pentagonal quasicrystals and concluded that the additional degrees of freedom associated with the hyperspace description imply the presence of additional diffusive modes. Later on, Kalugin and Katzexplicitly elaborated a scenario for long-range transport of matter by a phason-mediated mechanism in quasicrystals. Since then, numerous studies have been devoted to the search for experimental evidence for such a process. Experimental work has been carried out by different means such as conventional tracer diffusion, nuclear magnetic resonance studies, internal friction studies, magnetic susceptibility measurements and coherent X-ray scattering.
However, no direct observation of phason diffusion processes has been reported, and the interpretation of these experimental results remains a matter of debate. Therefore, the dynamics of phason defects is one of the open issues in quasicrystal physics today. In this paper we present for the first time a direct and quantitative investigation of phason diffusion performed by means of in situ heating studies using transmission electron microscopy (TEM).
Further reading: Acta Materialia 54 (2006) 3233-3240