An international scientific research team used the advanced neutron spectrometer and other facilities at the Laue-Langevin Institute in France to observe the strange form of water - "Plasticice VII" (Plasticice VII) for the first time. Studying the plastic state and superionic state of water can help people further understand the internal structure and glacier flow of icy satellites such as Ganymede and Callisto, as well as icy planets such as Uranus and Neptune. The relevant paper was published in the latest issue of Nature magazine.
Image source: Physicist Organization Network
Generally speaking, water has three basic forms: solid, liquid and gas. But in fact, water has many more forms, some of which only appear under high temperatures and pressures and are called exotic states. Fifteen years ago, scientists used molecular dynamics simulations to predict the existence of "plastic ice seven", and this study was the first to experimentally observe this form.
The plastic state is a mixed state that has properties of both solid and liquid states. Within Plastic Ice Seven, water molecules form a rigid cubic lattice - just like in Ice Seven (a cubic crystalline ice form that can be formed from liquid water above 3 GPa after cooling to room temperature), but at the same time exhibit picosecond rotational motion within liquid water. This peculiar form appears in a high temperature environment of 177℃-327℃ and a pressure environment of 0.1GPa-6GPa (1GPa=1 billion Pascal).
Quasi-elastic neutron scattering (QENS) technology played a key role in this research. QENS can detect the translational and rotational dynamics of objects more accurately than other spectroscopic technologies. Through QENS, the team identified three different forms that water takes on when temperature and pressure change: liquid water in which water molecules move both in translation and rotation; solid ice in which both translation and rotation are frozen; and "plastic ice seven" in the middle. Within Plastic Ice Seven, water molecules arranged into an ordered crystal structure lose the ability to translate freely but retain the ability to rotate.
Further analysis showed that the molecular dynamics of Plastic Ice 7 may be more complex than originally simulated, and that its molecular rotation mechanism differs from the previously predicted free-rotor behavior. Moreover, the transition of water from "ice seven" to "plastic ice seven" is continuous, which means that "plastic ice seven" may be the "predecessor" of another superionic state that exists at higher temperatures and pressures.