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Recent Study from the Theoretical Chemistry Laboratory: Exploration of the new ices

Posted on : July 5, 2018 | Modified on : July 26, 2018

The theoretical chemistry laboratory studies various phenomena related to water through statistical mechanics theory and computer simulation. Let's introduce studies on ice here.

Water becomes ice by cooling. In the ice, water molecules regularly make a hexagonal structure, which is reflected in the crystal structure of the hexagonal symmetry of snow flakes (it overlaps with itself when rotated by 60 degrees).

Can we make ice other than hexagon? Experiments have shown that ice crystals change to unusual crystal structures such as triangles and squares when the pressure is high. It is not high enough pressure even at the deepest part of the ocean, but it is thought that the strongly compressed ice near the outer planetary satellite and at the crustal mantle of the Earth has different crystal structure and properties from ordinary ice. For example, ultra-high pressure ice 7 is square (cubic), and the melting point exceeds 100°C.

It has been investigated throughout from low pressure to high pressure so far. The crystal structure of ice has increased by 5 kinds this century, and it is known that there are 17 kinds indeed.

Will it still continue to increase in the future? What temperature and pressure shall we search?

The crystal structure of ice has certain rules. The ice found so far satisfies the conditions such as (1) 4 hydrogen bonds are necessarily connected to one molecule (4-coordinated network) and (2) 3-dmiensional periodicity. Although the possible geometric structure satisfying these conditions is infinite, only a few structures can be produced in reality.

There may be other relatively stable ice crystal structures that have been missed in the experiments. Computer simulation that can answer this question. Structures that can not be realized by experiment for various technical reasons can be predicted in advance by using simulation.

In the theoretical chemistry laboratory, due to the activity of undergraduates and graduate students, we have predicted several new ice structures so far. Every crystal has a complicated and unexpected crystal structure which we would never expect in a method other than simulation. Some of them may be created in reality in the near future.

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