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. Central to this work has been understanding how thermodynamically predicted phase transitions manifest in experiment and in theoretical calculations, and how they can be detected. 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. Determining crystal structures under high pressure remains demanding even with modern X-ray and neutron diffraction — limited data often call for careful experimental design and creative analytical approaches. A growing focus of mine is measurement efficiency and automation. By building automated workflows in the laboratory and at KEK-PF BL-18C, I am working to acquire richer datasets that open the door to harder problems: temperature–pressure path dependence, metastable phase discovery, and structure determination under extreme conditions.
Recent news
- Aug. 2026I plan to present our work in IUCr 2026 in Calgary, Canada.
- 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
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

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.
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.