Dislocations in quasicrystals

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In quasicrystals, we can define a dislocation whose Burgers vector is a lattice translational vector of a 4D or higher dimensional lattice. Such a dislocation is a novel type of structural defect inherent to quasicrystal in the sense that it is accompanied not only by a conventional strain but also by a strain named phason strain which is caused by a lattice distortion in a complementary space orthogonal to real space. We performed the following researches for the purpose of clarifying the properties of such a dislocation. First, we perfomed plastic deformation experiments for icosahedra (i-) Al-Pd-Mn and decagonal (d-) Al-Ni-Co. For both samples, work softening, which is generally seen for quasicrystals, was observed. As a result of thermal activation analysis, we analyzed the data at various temperatures systematically on the assumption of temperature-dependent dragging stress for a dislocation. Second, we investigated structure and properties of phasons, which play an important role in the dynamical property of the dislocation in quasicrystals. Here, i) we observed the structure of a phason defect by STM, ii) we measured specific heat at high temperatures to investigate thermally equilibinium phasons and iii) we observed the phasons in motion at high temperatures by in-situ high-temperature HRTEM. In i), we observed a frozen-in phason defect in d-Al-Ni-Co by STM. In ii), we measured specific heat (Cp) of i-Al-Pd-Mn and d-Al-Cu-Co by DSC. we observed an upward deviation of Cp from the Dulong-Petit value of 3k/atom (k : Boltzman's constant) and it reached 5k at about 800℃. This can be interpreted as a thermal excitation of phasons. We calculated the entropy change from the increment of Cp and compared it with the result of the computer simulations previously done. In iii), we observed directly phasons in motion by HRTEM for the first time.