Hydrogen diffusion in titanium and zirconium hydrides

Quasicrystals are solids with long-range quasiperiodic order and orientational order with disallowed crystallographic symmetry (e.g. 5-fold). In addition to the point defects known from crystals (vacancies, interstitials, or antistructure atoms) quasicrystals contain phasonic defects (phasons), which represent violations of the matching rules. Phasons allow displacement of atoms without contribution of vacancies. In icosahedral AlPdMn single quasicrystals two internal friction peaks are observed. Peak A at 370 K (f=3 Hz) has an activation enthalpy of 0.98 eV and a frequency factor close to the Debye frequency (2×1015 s-1) and is therefore attributed to point defects, i.e. atomic jumps enabled by phason flips or vacancies. Peak B at 870 K (f=3 Hz) is determined by an activation enthalpy of 4 eV and an abnormal high frequency factor (3×1024 s-1), is much broader than a simple Debye-peak, and highly asymmetric. The temperature variation of the relaxation time exhibits significant deviation from ideal Arrhenius behaviour for higher temperatures (T>920 K) indicating a sort of phase transition. Peak B can be attributed to a relaxation process in which dislocation movement is controlled by creation and movement of phason defects.