Pressure-Induced Phase Transformation of Quasicrystals

点击翻译按钮获取中文摘要

Since the discovery of quasicrystals with an icosahedral symmetry, there has been a great deal of effort to investigate their stability or structure. Quasicrystals were obtained as metastable phase (i-Al-Mn) at first, but now are synthesized also as thermodynamically stable phase (i-Al-Li-Cu, i-Al-Ru-Cu). To investigate the stability of quasicrystals, we have performed x-ray diffraction experiment under high pressure.For Al-Mn and Al-Ru-Cu, the i-phase was isotropically compressed and stable up to 23 and 30 GPa, respectively. On the other hand, in Al-Li-Cu i-phase, a successive phase transformation was observed to occur in two stage under high pressure. The first is from the i-phase to an amorphous state. The transition starts at the pressure of around 10 GPa, and the i-phase and the amorphous phase coexist over a wide pressure range. After disappearance of the i-phase, the second stage of transition from the amorphous to a long-range ordered state was caused by application of pressure of around 28 GPa.To reveal details of the transition from the i-phase to the amorphous phase, we measured the dependence of x-ray diffraction peak widths on pressure. Peak widths of all lines become broader with an increase in pressure but the increment of the peak widths exhibited no clear systematic dependence on momentum transfer (G_*) nor phason momentum (G_*). Such a G-independent broadening of x-ray peaks suggests that a fragmentation of the structure takes place on increasing the pressure. The fragmentation of the structure may possibly be realized by producing variants of small grains of approximate crystals by introducing phasons. High temperature experiments were also performed at the pressure where the i-phase and the amorphous phase coexist. At around 100 ﾟC, the i-phase was more stable than the amorphous phase, but at around 250 ﾟC, the i-phase turned into crystal phase.