超高圧下における水の構造と水素結合

水是我们生命中最重要的液体，它也因其异常的性质而引起人们的关注。本文采用x射线衍射实验研究了超高压条件下水的结构。通过使用钻石容器和贵金属瓶盖，我们成功地扩大了测量的压力范围。能量色散x射线衍射数据在弹簧-8的JAEA光束线BL14B1上用立方型压机测得最高为9.1 GPa和423℃，在弹簧-8的公共光束线BL04B1上用kawai型压机测得最高为17.1 GPa和572℃。在环境压力下，由于氢键的存在，液态水的网络结构仍然存在。因此配位数约为5，远小于简单液体的配位数约为10。高压下液态水的结构变化总结如下：(1)常压下，液态水的结构因子S(Q)具有特征形状。在2和3 A^<-1>处有两个小峰。随着压力的增加，第一个峰的强度增加，并向更高的q偏移，两个峰合并形成一个强的单峰。在5 GPa以上，S(Q)可以用硬球模型很好地描述。(2)水-水最近邻距离在4 GPa以内没有变化，而配位数迅速增加。在4 GPa的情况下达到10。在此压力以上配位数基本不变，而最近邻距离减小。(3)在4 GPa以上，水体中最近邻距离的减小与具有BCC结构的冰七的最近邻距离减小相似。这表明水的结构在这个压力范围内均匀收缩。本研究明确了水的结构变化机制随压力的变化而变化。

Water is the most important liquid for our life and it also attracts attention due to its anomalous properties. In this study, structure of water under ultra high pressure was studied by X-ray diffraction experiments. By using a diamond container and a noble metal cap, we have succeeded in extending the pressure range of the measurements. Energy-dispersive X-ray diffraction data were taken up to 9.1 GPa and 423℃ using a cubic-type press on JAEA beamline BL14B1 at SPring-8 and up to 17.1 GPa and 572℃ using a Kawai-type press on public beamline BL04B1 at SPring-8. At ambient pressure, a network structure due to hydrogen bonds remains in liquid water. Therefore coordination number is about 5,which is much smaller than about 10,the value for simple liquids. The structural changes under high pressure are summarized as follows, (1)At ambient pressure, structure factor, S(Q), of liquid water has a characteristic shape. There are two small peaks at 2 and 3 A^<-1>. With increasing pressure, the first peak increases in intensity and shifts to higher Q. The two peaks merge and form a strong single peak. Above 5 GPa, S(Q) can be well described by a hard-sphere model. (2)The water-water nearest neighbor distance does not change up to 4 GPa whereas the coordination number increases rapidly. It reaches to about 10 at about 4 GPa. The coordination number is almost constant above this pressure while the nearest neighbor distance decreases. (3)Above 4 GPa, the decrease of the nearest neighbor distance in water is similar to that of the nearest neighbor distance of ice VII, which has BCC structure. It suggests that the structure of water uniformly contracts in this pressure range. This study clearly indicates that the mechanism of structural change of water changes with pressure.