Routes to cubic ice through heterogeneous nucleation

Abstract

The freezing of water into ice is one of the most important processes in the physical sciences. However, it is still not understood at the molecular level. In particular, the crystallization of cubic ice (Ic)-rather than the traditional hexagonal polytype (Ih)-has become an increasingly debated topic. Although evidence for Ic is thought to date back almost 400 y, it is only in the last year that pure Ic has been made in the laboratory, and these processes involved high-pressure ice phases. Since this demonstrates that pure Ic can form, the question naturally arises if Ic can be made from liquid water. With this in mind, we have performed a highthroughput computational screening study involving molecular dynamics simulations of nucleation on over 1,100 model substrates. From these simulations, we find that 1) many different substrates can promote the formation of pristine Ic; 2) Ic can be selectively nucleated for even the mildest supercooling; 3) the water contact layer's resemblance to a face of ice is the key factor determining the polytype selectivity and nucleation temperature, independent of which polytype is promoted; and 4) substrate lattice match to ice is not indicative of the polytype obtained. Through this study, we have deepened understanding of the interplay of heterogeneous nucleation and ice I polytypism and suggest routes to Ic. More broadly, the substrate design methodology presented here combined with the insight gained can be used to understand and control polymorphism and stacking disorder in materials in general.