|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1992 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1991 : ¥1,100,000 (Direct Cost : ¥1,100,000)
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.
このアモルファス相への転移はスラギッシュであり,広い圧力に亘って両相が共存した。この転移を更に詳しく調べるためX線回折線巾の圧力依存性を調べた。回折線巾は圧力の増加につれて,顕著に増加したが,その増加がphason momentumとの相関がないため,アモルファス化は小粒経の近似結晶への転移の可能性が示唆された。再に,このI-相とアモルファス相の共存圧力下で高温実験を行った。100℃程度の温度では回折線巾は増加し,I-相の方が安定化するという結果を得た。すなわちI-相とアモルファス相との相境界は正のスロープを持っている。再に300℃程度まで昇温するとI-相は消失し,結晶相へと転移した。この結晶相が単一相であるか否か等現在解析中である。また,準結晶と同様分子自身が五回対称を持つC_<60>等フラーレンの構造変態も調べた。 Less