Physica A 304 (2002) 43–52 www.elsevier.com/locate/physa Jumping between water polymorphs M.A. Ricci a ; ∗ , A.K. Soper b; c a Dipartimento di Fisica “E. Amaldi”, Universit a di Roma III, and INFM, Unit a di Roma Tre. Via della Vasca Navale 84, 00146 Roma, Italy b ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK c Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK Abstract The three site–site partial structure factors of two polymorphs of water have been extrapolated from neutron diraction experiments exploiting the isotopic H=D substitution performed on pure supercooled water. The low density polymorph has an open hydrogen-bonded structure, while the high density polymorph has non-tetrahedral O–O–O angles and a collapsed second coordination shell, which implies broken hydrogen bonds between the rst and second neighboring shells. c  2002 Elsevier Science B.V. All rights reserved. PACS: 61.20.Qg; 61.12.-q; 61.25.-f Keywords: Water; Neutron diraction 1. Introduction Liquid polymorphism for a pure, isotropic substance does not fall within our common experience. Nevertheless, recent studies on a variety of liquids [1–8] have revealed a thermodynamic behavior that suggests the occurrence of multiple liquid phases distin- guished by density, not chemical composition. Such liquid–liquid phase transitions can be evidenced only when a wide range of densities is explored, thus implying the need for high pressure technology, that is becoming available only in recent years. More- over, in most liquids high pressures can be achieved only at high temperatures, and require an even more sophisticated technology. When, on the contrary, the transition is predicted at low temperatures, as for instance in water, it can be obscured by crystal nucleation or glass transition. The most promising candidates to observe such transition are liquids having an open molecular coordination environment at low pressure: water, * Corresponding author. Tel.: +39-06-55-17-72-26; fax: +39-06-55-79-303. E-mail address: riccim@s.uniroma3.it (M.A. Ricci). 0378-4371/02/$-see front matter c  2002 Elsevier Science B.V. All rights reserved. PII:S0378-4371(01)00516-7