Author: Lu Dabiao | University of Chinese Academy of Sciences
Training unit: Institute of Physics, Chinese Academy of Sciences
Review: Pan Zhao | Associate Professor, Institute of Physics, Chinese Academy of Sciences
In this universe in which we live, the existence of ice is universal and mysterious. Ice is everywhere, from the inner planets of the solar system and the poles of comets, to the distant moons of gas giants and dwarf planets. And on the earth beneath our feet, one of the most common phase transitions is the formation of "ordinary ice" by water, and when winter comes, snowflakes fall, it is an unforgettable natural feast.
However, under the microscope of science, ice is not as simple as it seems. It is a highly complex material that has been at the forefront of materials science research. For example, in order to reveal how ice formation is inhibited in living organisms, Jacques Dubochet developed cryo-electron microscopy, a groundbreaking research that earned him the Nobel Prize in Chemistry in 2017.
The human search for ice began in the early 20th century. Bridgman used high-pressure technology to reveal five different forms of ice for the first time. Since then, scientists have conducted in-depth research in this area, and to date, more than 20 different forms of ice have been discovered. The ices were named "Ice 1" through "Ice 19" in the order in which they were found.
Attentive friends may have noticed that there are only 19 names from "Ice 1" to "Ice 19", but there are more than 20 forms of ice. This is because some names cover multiple forms. Taking "Ice 1" as an example, it actually contains two different forms, namely "Ice 1h" and "Ice 1c". Among them, "Ice 1h" is the most common "common ice", and its structure is hexagonal. "Ice 1c" is cubic ice, and its structure is Cubic.
Cubic ice
In spring and summer, occasional solar halos occur due to the refraction or reflection of sunlight by ice crystals as it passes through cirrostratus clouds. The structure of ordinary ice is hexagonal, so the angle of view of the halo radius is usually 22 degrees and 46 degrees. However, in extremely rare cases, a 28-degree solar halo occurs, which is known as Scheiner's halo. The reason for its formation is the mysterious cube ice.
In 2023, Bai Xuedong's research group at the Institute of Physics revealed the mechanism of cubic ice formation. Using techniques such as in situ cryo-EM, they found that cubic ice first nucleated and grew at some heterogeneous interfaces. However, with the increase of the growth time of ice crystals, the proportion of hexagonal ice gradually increased. Therefore, although most of the snowfall commonly seen in nature is the condensation and growth of water molecules on surfaces such as dust minerals, cubic ice can occur at these heterogeneous interfaces. But as the ice crystals grow, these cubic ices gradually transform into the most common hexagonal ice.
"Ice 7" in diamonds
Last winter, the splendid scene of "splashing water into ice" in the northern region was impressive. "Splashing water into ice" generally requires hot water at a higher temperature. The water vapor evaporated from the hot water quickly condenses into Xiaoice crystals. So how long does it take to form a beautiful snowflake? Here's a GIF that shows Professor Libbrecht growing snowflakes in his lab. The snowflake is about 2.5mm long and takes 44 minutes to grow. If we want to make the freezing process faster, lowering the temperature is certainly one way to do it. In addition, increasing the pressure is another more effective method.
For example, at a pressure of 3 GPa (1 GPa = 109 Pa), water will condense into "ice7". This pressure is roughly equivalent to putting a small airliner on a fingernail. Compared to normal ice, Ice 7 has a very short setting time of only 6 nanoseconds, which is 11 orders of magnitude faster than the time it takes Professor Libbrecht to grow snowflakes in the lab.
In fact, deep in the earth's crust, there is "ice7". More interestingly, a 2018 article published in Science proved that some diamonds were sealed with "Ice 7".
Frozen Flame: "Ice 18"
If the pressure continues to increase to 300 GPa, it is possible to form "Ice 18". In the structure of "Ice 18", the huge pressure causes the covalent bonds inside the water molecule to break. Oxygen ions are forced to form a densely packed face-centered cubic structure, while protons (hydrogen ions) can only be located in the voids of oxygen ions. Moreover, hydrogen ions can move freely in these voids, making Bing 18 conductive as a metal.
The conductivity of "Ice 18" means that it has no band gaps, so "Ice 18" is not transparent like regular ice. In addition, the free-moving hydrogen ions also give Bing 18 a high melting point and remains solid at high temperatures close to 3000°C. This makes "Ice 18" a veritable "frozen flame".
According to NASA Voyager 2, the magnetic field of an ice giant like Uranus is very different from the dipole field of Earth and other planets. These planets have strange non-axisymmetric, non-dipole magnetic fields. It is entirely possible that "Ice 18" formed at the pressure and temperature of such planets, and its special electrical conductivity can even explain the special magnetic fields in ice giants.
Ice at higher pressures?
With the deepening of scientific research, human understanding of ice is also constantly enriched. From the cube ice to the "Ice 7" in diamonds to the frozen flame "Ice 18", every form of ice reveals the wonder and diversity of ice. As for ice at higher pressures, scientists are still exploring and researching. We look forward to more discoveries in the future that will give us a deeper understanding of the wonderful matter in this universe.
Bibliography:
- Christoph G. Salzmann; Advances in the experimental exploration of water’s phase diagram. J. Chem. Phys. 150, 060901 (2019).
- Huang, X. et al. Tracking cubic ice at molecular resolution. Nature 617, 86–91 (2023).
- O. Tschauner et al. Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle. Science 359,1136-1139(2018).
- Millot, M. et al. Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, 251–255 (2019).
- Libbrecht
Editor: Mu Zi