How Does the Structure of a Liquid Change When Pressure Is Applied?

In a crystal, atoms are arranged periodically. This is described as having long-range periodic order. Just as ice turns into water when its temperature is raised, a crystal turns into a liquid when its temperature is increased. The volume change at this point is only a few percent, so the average distance between atoms remains almost the same, but the arrangement of atoms loses its long-range periodicity.

So, what happens when pressure is applied to a substance, reducing its volume? In a crystal, the entire structure contracts homothetically. Then, in many cases, at a certain transition pressure, the entire crystal undergoes a phase transition, discontinuously changing into a different crystal structure.

In typical semiconductors like silicon and germanium, crystals with a diamond-type structure undergo a phase transition under ultra-high pressures of several hundred thousand atmospheres, discontinuously changing to the same crystal structure as tin. The high-pressure phase is metallic and exhibits superconductivity at low temperatures. In this way, the high-pressure phases of substances have properties completely different from those at one atmosphere. In this sense, high-pressure experiments can be described as research that continuously creates new materials.

What happens when pressure is applied to a liquid? Unlike in crystals, there is a distribution (structural fluctuations) in the distances between atoms, with some regions being easier to compress and others more difficult. For this reason, the way the local structure of a liquid contracts is thought to be significantly different from that of a crystal (Fig. 1).

Since pressure is expressed as force per unit area, high-pressure experiments require large forces or small samples. We use a 1500-ton press or samples as small as 0.1 mm. The study of the structure of liquids under ultra-high pressure requires high-intensity, highly penetrating X-rays.

At synchrotron radiation facilities such as SPring-8 in Hyogo Prefecture (Photo 1) and the High Energy Accelerator Research Organization (KEK) in Ibaraki Prefecture, we can use X-rays that are more than a million times more intense than those in a laboratory. Using these, we have achieved many results, including observing rapid changes in the local structure within liquids and discovering local structures unique to liquids that are not seen in crystals.