Crystallography of ice

Why is it interesting to study ice? The book reviewed in this issue 4 of Volume 28 of Crystallography Reviews boosts the reader’s amazement, enthusiasm and curiosity about the wonder and science of ice and snowflakes. While the review article gives a comprehensive overview of all the known twenty polymorphic forms of ice with an outlook to further predictable forms.

Hydrogen is the most common element in the Universe, around 75% of all atoms in our galaxy is hydrogen. Oxygen is the third most common element in space, making up about 1% of all the atoms. Water, made of these two elements, plays a key role in the formation and evolution of our planetary system and it is essential in the life on Earth.

The planet Earth is formed in the warm part of the sun’s protoplanetary disk, at a location well within the ‘snow line’. The presence of water is vital in the search for extraterrestrial life. Brighter regions observed by an optical telescope could indicate reflections of frozen water. Landers and rovers can collect samples from the surface of a planet to be placed in an analysis chamber. There is water-ice on the surface of the moon near the poles [1]. A subglacial lake on Mars, 1.5 km below the southern polar ice cap was detected [2]. Asteroids in the asteroid belt also contain large amounts of water-ice that could be harvested if humans ever regularly travel beyond the inner Solar System. Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, have huge subsurface oceans with a layer of tens or hundreds of kilometres of ice covering their surfaces. Other moons of Jupiter and Saturn such as Ganymede and Titan may have subsurface oceans as well. There are very likely still millions of other icy bodies out there, just waiting to be explored. The varying conditions on the myriads of planets, moons, asteroids in space provide the opportunity of the probable formation of different ice polymorphs.

Although ice may be one of the most studied crystalline solids in human history, new discoveries on ice are still being reported on a regular basis. The full review article ‘Neutrons meet ice polymorphs’ by Kazuki Komatsu from the Geochemical Research Center, Graduate School of Science, The University of Tokyo, Japan, gives an extensive review of the discovery and crystallographic characterization of ice polymorphs formed in different conditions. The presented historical background elucidates the experimental difficulties in ice research. The current epoch described by the author is the ‘age of ice-rush’, as the rate of discovery of ice polymorphs has accelerated in the last two decades owing to the advances in neutron diffraction studies of ice under pressure. The most extreme conditions, of both high-temperature and high-pressure, led to a new ice polymorph being created (namely XVIII) at 100 GPa and 2000 K. The transition between hydrogen-ordered and hydrogen-disordered phases is a common problem for many ice polymorphs. Ice polymorphs may exist in fully ordered and fully disordered states, but also they can be in partially-ordered states in between. The review by Kazuki Komatsu consists of three main topics. The first part presents how recent technological developments for high-pressure neutron diffraction have contributed to the understanding of ice polymorphs. The development of the pressure-temperature variable system for neutron scattering (Mito system; https://sites.google.com/site/daclabingcrc/j-parc/mito-system) played a crucial role in the exploration of the vast and strange wilderness for ice polymorphs, including its metastable or non-equilibrium regions. High pressure anvil cells are also depicted, which are used to achieve very high pressures and to perform single crystal diffraction experiments. In the next chapter we get an overview of the immensely complex phase diagram of ice and its classifications. The third and most extensive part of the review goes through each polymorphic ice form one by one. It discusses the current understanding, as well as the unanswered questions for respective polymorphs. The multiple amorphous states of ice such as low-, high-, and very-high-density amorphous ices and their polyamorphic phase transitions have been already reviewed and which the author references. Finally, the author predicts ice polymorphs, which will be discovered in the future. The prediction is based on melting experiments, observation of phase transitions, as well as computer simulations. For the experimental evidence of new phases, a technical breakthrough is required in the application of powder and single crystal neutron diffraction, spectroscopies or physical property measurements. The boundary between what is known and what is unknown about ice is like the complexities of a snow crystal, as Kazuki Komatsu points out. This connects well with our book review in this issue.

The book ‘Snow crystals: a case study in spontaneous structure formation’ by Kenneth G. Libbrecht, Princeton University Press, Princeton, 2022, was reviewed by Vesselin Tonchev from the Department of Meteorology and Geophysics, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’ in Bulgaria and Valeria Stoyanova from Institute of Physical Chemistry ‘Acad, Rostislaw Kaischew’, the Bulgarian Academy of Sciences. The book reviewed describes the current state of research on the subject, and not only for the specialized audiences of materials science and meteorology. Countless kinds of snowflake patterns with astonishing symmetries materialize under different atmospheric conditions when crystalline ice grows from water vapour. In addition to the shapes visually familiar to us a rich variety of lesser-known forms: needle clusters, hollow columns, bullet rosettes, triangular crystals, and exotic capped columns etc. appear. The book introduces us to a great variety of topics: why and how the snowflakes grow, the nucleation, the physics of crystal growth, the crystalline ice structures, molecular dynamics at the ice surface, diffusion-limited growth, surface attachment kinetics, as well as the computational models of snow crystal growth, and the laboratory techniques for creating and studying snow crystals. The book provides an extensive scientific insight into the formation of the hieroglyphs from the Sky. The reviewers of the book are favourably impressed.

As ever we welcome new ideas for review articles, and for suggestions regarding books to be reviewed. Please contact me at the e-mail address below. I look forward to welcoming your submissions.