Modelling the atomic structure of very high-density amorphous ice

TL;DR

This study models the structure of very high-density amorphous ice by disordering crystalline phases, successfully reproducing experimental structural features and providing insights into its composition.

Contribution

It introduces a novel disordered-crystal modeling approach for VHDA ice based on crystalline phases, matching experimental data.

Findings

Excellent fit to pair-correlation function g(r)

Reproduces low-k behavior of structure factor S(k)

Predicts phase composition consistent with experiments

Abstract

The structure of very high-density amorphous (VHDA) ice has been modelled by positionally disordering three crystalline phases, namely ice IV, VI and XII. These phases were chosen because only they are stable or metastable in the region of the ice phase diagram where VHDA ice is formed, and their densities are comparable to that of VHDA ice. An excellent fit to the medium range of the experimentally observed pair-correlation function g(r) of VHDA ice was obtained by introducing disorder into the positions of the H2O molecules, as well as small amounts of molecular rotational disorder, disorder in the O--H bond lengths and disorder in the H--O--H bond angles. The low-k behaviour of the experimental structure factor, S(k), is also very well reproduced by this disordered-crystal model. The fraction of each phase present in the best-fit disordered model is very close to that observed in the probable crystallization products of VHDA ice. In particular, only negligible amounts of ice IV are predicted, in accordance with experimental observation.