Unexpectedly, surfaces may help freezing tetrahedral liquids like silicon, germanium and water—and this has to do with their density variation upon freezing: these systems expand when they solidify, instead of contracting, as most substances do.

Melting and freezing are phenomena that we all experience in everyday life—for example freezing of water into ice-- yet their understanding at the  atomistic level is very challenging. In fact experimental techniques available at present cannot really “see” what happens at the atoms of a substance when freezing occurs.

A new computational study predicts what happens at the microscopic level when you freeze silicon and germanium, two common semiconductors. The results are presented in a paper entitled  “Surface-induced crystallization in supercooled tetrahedral liquids,”  published Aug. 9 in the early online edition of the journal Nature Materials (http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat2508.html). The study was led by Prof. Giulia Galli at UCD and was done in collaboration with Tianshu Li and Davide Donadio, collaborators of Prof. Galli in the Angstrom group (http://angstrom.ucdavis.edu/), and Luca Ghiringhelli at the Fritz Haber Institute of the Max Planck Society in Germany.  

Unlike melting, that is normally observed at surfaces, freezing usually originates in the bulk of a material.  The new study reports that in supercooled silicon and germanium, the presence of surfaces does help the freezing process (it makes it more probable and thus faster), contrary to what happens in most systems. The nucleation mechanism proposed  by Galli and collaborators could prove to be relevant for other tetrahedrally coordinated systems, in particular water in the atmosphere.

The main factor about surface-induced nucleation is the way temperature varies with pressure in Si and Ge, close to the freezing point.  Silicon, Germanium and water shows a variation of temperature with pressure which is opposite to that observed in the great majority of simple materials. And this is related to what happens to their density upon freezing: they expand, instead of contracting, as most materials do.