Hiroki Kobayashi
PhD Student · The University of Tokyo
Ice
The striking polymorphic diversity of a single simple molecule is what first drew me to ice research. My master and PhD research have centered on the exploration of ice polymorphs, with a particular focus on hydrogen-ordered and metastable phases. More recently, my interests have extended to the high-pressure ices likely found deep within icy moons.
High-Pressure Crystallography
My interests extend beyond ice to molecular crystals more broadly, and crystallography under extreme conditions is a fundamental aspect of this research. Recently, I am particularly interested in automation of experiments. By automating the entire process from experimental control to data analysis, I aim to explore a wider range of conditions more efficiently.
Recent news
- Aug. 2026I plan to present our work in IUCr 2026 in Calgary, Canada.
- Jul. 2026My work on crystallization and ordering of ice IV is published in J. Phys. Chem. C. H.K. et al., JPCC (2026)
- May 2026I presented our work on ices XXI, XXII, and XXIII in Water-X 2026 in La Maddalena, Italy.
- Feb. 2026I finished 1-year stay at IMPMC, Sorbonne Univ., Paris and returned to Tokyo.
- Dec. 2025My work on the NLC and pressure-induced phase transition in crystalline purine is published in Chem. Phys. Lett. H.K. et al., CPL (2025)
- Aug. 2025I presented our work on crystallization of amorphous calcium carbonate at the European Crystallographic Meeting (ECM) in Poznań, Poland.
- Jul. 2025I reported as a preprint three new phases of ice and their structure. H.K. et al., arXiv (2025)
- Feb. 2025I received the Ludo Frevel Crystallography Scholarship Award from ICDD. "In-depth neutron diffraction study of unusual hydrogen ordering behaviour in ice VI" (2,500 USD)
- Mar. 2024I received the Research Encouragement Award of the Graduate School of Science, The University of Tokyo, and was nominated as the representative of the Graduate School of Science.
- Nov. 2023My work on weak hydrogen ordering in ice IV is published in J. Phys. Chem. Lett. H.K. et al., JPCL (2023)
- Sep. 2023I presented our work on hydrogen ordering in ice IV at PCI 2023 in Sapporo, Japan.
- Aug. 2023I presented our work on hydrogen ordering in ice IV at IUCr 2023 in Melbourne, Australia.
- Mar. 2022I received the Department of Chemistry Award, Faculty of Science, The University of Tokyo.
Featured Works
Ice IV from Supercooled Water: Reproducible Crystallization, Structure, and Ordering
The Journal of Physical Chemistry C (2026) · DOI:10.1021/acs.jpcc.6c00737
Densification processes of crystalline purine: Negative linear compressibility and pressure-induced phase transition
Chemical Physics Letters 884:142600 (2025) · DOI:10.1016/j.cplett.2025.142600
We studied pressure-induced structural changes in crystalline purine (C5H4N4). The ambient phase persists up to about 0.7 GPa until it transforms into a newly characterised high-pressure phase with a completely different packing structure. We pointed out that the intermolecular hydrogen bonds may be doubled, for which further spectroscopic/theoretical corroboration is desired. If so, we suggest this phase transition is characteristic in purine but not expected in pyrimidine, which has no ability to double the hydrogen bonds.
Slightly Hydrogen-Ordered State of Ice IV Evidenced by In-Situ Neutron Diffraction
The Journal of Physical Chemistry Letters 14:10664 (2023) · DOI:10.1021/acs.jpclett.3c02563
We reported crystallographic evidence for the hydrogen ordering in ice IV by neutron diffraction. It shows discontinuous behaviour at 120 K and Rietveld analysis at the low-T side suggests it is a very weak hydrogen ordering structural transition.
Ice IV, a metastable high-pressure phase of water nicknamed the "will-o'-the-wisp," has long lacked a reproducible synthesis route. We show that ice IV can be reproducibly crystallised from deeply supercooled water at the homogeneous nucleation temperatures using emulsified water. This work led to our important discovery of ice XXI-XXIII, and I’m happy to have finally published it.