Roles of local structural ordering in anomalies and crystallization of water

The anomalous thermodynamic and kinetic behaviour of water is known to play a key role in many chemical, biological, geological and terrestrial processes. However, its origin is still much debated despite decades of intense research. An early insight, which dates back to Rontgen, is that the complexity of water may be modelled by a mixture of two structural motifs, but the idea has found limited support due to the lack of microscopic evidence. Unlike assuming the presence of two structural motifs, we proposed a two-order-parameter model of liquid [1,2], which focuses on the (cooperative) formation of locally favoured structures in a sea of random liquid structures. Here we provide a simple physical description of water anomaly from microscopic data obtained through computer simulations. We introduce a novel structural order parameter, which quantifies the degree of translational order of the second shell, and show that this order parameter can accurately characterize the state of water [3]. A two-state modeling of these microscopic structures [1,2] is used to describe the density and compressibility anomalies, and being compatible with the existence of a second critical point in the deeply supercooled region. Furthermore, we reveal that locally favored structures in water not only have translational order in the second shell, but also contain five-membered rings of hydrogen-bonded molecules. This suggests their mixed character: the former helps crystallization, whereas the latter causes frustration against crystallization [3]. We find that this local structural ordering plays a key role in ice crystallization of a deeply supercooled water, through a novel metastable ice crystal phase, which we named Ice 0 [4]. We also discuss the glass-forming ability of water-type tetrahedral liquids, focusing on the friendliness of local structural orderings with a crystal structure to be nucleated. This work is performed in collaboration with John Russo and Flavio Romano.

[1] H. Tanaka, Thermodynamic anomaly and polyamorphism of water, Europhy. Lett. 50, 340 (2000).
[2] H. Tanaka, Bond orientational order in liquids: Towards a unified description of water-like anomalies, liquid-liquid transition, glass transition, and crystallization, Eur. Phys. J. E 35, 113 (2012).
[3] J. Russo and H. Tanaka, Understanding water's anomalies with locally favored structures, Nat. Commun. 5, 3556 (2014).
[4] J. Russo, F. Romano, and H. Tanaka, New metastable form of ice and its role in the homogeneous crystallization of water, Nature Mater. 13, 733 (2014).