Pressure-Induced Phase Transformation of Quasicrystals

自从具有二十面体对称性的准晶被发现以来，人们对其稳定性或结构进行了大量的研究。准晶最初是作为亚稳定相(I-Al-Mn)合成的，现在也是以热力学稳定相(I-Al-Li-Cu，I-Al-Ru-Cu)的形式合成的。为了研究准晶的稳定性，我们在高压下进行了X射线衍射实验，对于Al-Mn和Al-Ru-Cu，I相是各向同性压缩的，并分别稳定到23 Gpa和30 Gpa。而在Al-Li-CuI相中，在高压下观察到两个相变阶段。第一个阶段是从I相到非晶态。相变开始于10 Gpa左右的压力，在较宽的压力范围内，I相和非晶相共存。在I相消失后，由非晶态向长程有序态转变的第二阶段是由28 Gp左右的压力引起的。为了揭示从I相向非晶态转变的细节，我们测量了X射线衍射峰宽随压力的变化关系。所有谱线的峰宽都随着压强的增加而变宽，但峰宽的增加与动量传递(G_*)和相子动量(G_*)没有明显的系统依赖关系。这种与G无关的X射线峰展宽表明，随着压力的增加，结构发生了碎裂。结构的碎裂可能是通过引入相干子产生近似晶体的小颗粒的变体来实现的。高温实验也是在I相和非晶相共存的压力下进行的。在100ﾟC左右，I相比非晶相更稳定，但在250ﾟC左右，I相转变为晶相。

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.