Order and disorder in crystalline ice explained

A fascinating substance with unique properties, ice has intrigued humans since time immemorial. Unlike most other materials, very low temperature ice is not as ordered as it could be. A collaboration between the Scuola Internazionale Superiore di Studi Avanzati (SISSA), the Abdus Salam International Center for Theoretical Physics (ICTP), the Institute of Physics Rosario (IFIR-UNR), with the support of the Istituto Officina dei Materiali of the Italian National Research Council (CNR-IOM), has introduced new theoretical interventions in the reasons why this is happening and how part of the missing order can be recovered. In that ordered state, the team of scientists has described a relatively obscure yet fundamental property of very low temperature ice: ferroelectricity. The results, published in PNAS, are likely to extend to ice surfaces, a possibility that could be relevant for the agglomeration of ice particles in interstellar space.

“In an ideally ordered piece of ice, the hydrogen atoms of each water molecule must point in the same direction, like soldiers in a platoon look ahead,” explains Alessandro Laio, physicist at SISSA and ICTP. If that were the case, ice would show macroscopic electrical polarization – it would be ferroelectric. Instead, even at very low temperatures, water molecules in ice behave like unruly soldiers, all looking in different directions. ‘

This abnormal behavior, discovered experimentally in the 1930s, was immediately and famously explained by Linus Pauling: the lack of discipline is a result of the ‘ice rule’ restriction – each oxygen atom would have to have two and only two protons at any one time to to make H.₂O. The difficult kinetics created by that constraint causes the ordering process to become infinitely slow, as in a platoon where each soldier had four neighbors and had to keep two hands on the shoulders of two of them.

“ Were it not for impurities or defects, which appeared to play a revealing role, one would still not know today whether the proton order and ferroelectricity of bulk crystalline ice is a real possibility or a fabrication, since neither neither experiments nor simulations that overcome the kinetic retardation caused by ice rules, ”said Erio Tosatti, physicist from SISSA, ICTP and CNR-IOM Democritos.

Impurities, such as a KOH that replaces H.₂Oh, in fact, they are known to order nucleation and ice formation and ferroelectricity at very low temperatures, although only partial and slow. It was once again suspected that the ‘ice rule’ was behind the slowness of this process, but exactly how that worked was not really known.

Together with Jorge Lasave and Sergio Koval from IFIR-UNR in Argentina, both ICTP associate members, Alessandro Laio and Erio Tosatti designed a theoretical model and strategy to explain the behavior of both pure and doped ice cream.

According to this model, the scientists explain, once an impurity is introduced into an initial unbalanced low-temperature, disordered state, it acts as a seed for the ordered phase, but in a peculiar way: not all ‘soldiers “Start looking around the impurity in the right direction, but only those in front of or behind the impurity. So at the end of the process, only a series of soldiers within the platoon will be ordered.” This very atypical process has many of its features. which may explain the slow and incomplete onset of the ferroelectric order in true doped ice.

“Although the study is currently limited to bulk ice,” conclude Tosatti and Laio, “the mechanism highlighted is likely to extend to ice surfaces, where arrays of ordered protons could nucleate at low temperatures, generating a long-known small amount of local ferroelectricity. polarization, a phenomenon also mentioned as possibly relevant to the agglomeration of ice particles in interstellar space. ”