Direct experimental evidence of phonon–phason coupling in an Al-Pd-Mn icosahedral quasicrystal

Due to their unique structural order, quasicrystals have special elastic degrees of freedom called the phason degrees of freedom, in addition to the conventional phonon degrees of freedom. In this study, we demonstrate the presence of phonon–phason coupling, expected from the generalized elastic theory of quasicrystals, in an Al-Pd-Mn icosahedral quasicrystal by a new experimental method that probes directly the elastic free energy function of the QC. Conventional phonon strain was applied to the quasicrystal at a high temperature where phasons are active. In response to the applied phonon strain, phason strain was observed to be induced by X-ray diffraction measurements, which demonstrated clearly the presence of phonon–phason coupling. Then, the coupling strength was quantitatively evaluated by comparing the magnitude of the induced phason strain with the theoretical values. In view of the evaluated coupling strength, the origin of an internal friction peak observed previously for an Al-Pd-Mn icosahedral quasicystal and the possible effects of the coupling on the onset of phasonic elastic instability are discussed.