超高圧下における水の構造と水素結合

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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.