1. Department of Chemistry, Fukuoka University of Education, Fukuoka 811-4192, Japan
2. Department of Chemistry and Center for Materials and Analysis, University of Nebraska, Lincoln, Nebraska 68588, USA
3. Department of Chemistry, Okayama University, 3-1-1, Tsushima, Okayama 700-8530 Japan
4. Present address: Department of Chemistry, U.S. Naval Academy, Annapolis, Maryland 21402, USA

Correspondence to: Kenichiro Koga¹ Correspondence and requests for materials should be addressed to K.K. (e-mail: Email: kenkoga@fukuoka-edu.ac.jp).

Following their discovery¹, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary^{2, 3, 4, 5, 6, 7}, mould^{8, 9, 10, 11} or template^{12, 13, 14} in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions¹⁵. Particularly intriguing is the conjecture¹⁶ that matter within the narrow confines of a carbon nanotube might exhibit a solid–liquid critical point¹⁷ beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid–liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.