Science Online: Why does water freeze while silica forms glass?

Everyone knows that water freezes at zero degrees Celsius. However, the "cousin" of water, silica, exhibits unstable behavior during the cooling process. When liquid silicon is cooled, its atoms cannot be arranged. Ordered crystal. The resulting solidified substance-a mechanical solid, but a microscopic liquid-is a glass.

Water is composed of individual H2O molecules, which are combined by strong covalent bonds, but interact through weaker hydrogen bonds. The stable molecular structure of water restricts the freedom of atoms, resulting in high directionality of water. Silica, however, does not have a molecular form, and the atoms are bound in a less directional manner, resulting in poor directionality.

Silica is one of the most abundant compounds on Earth, so it has a significant impact on glass-forming preferences. In some respects, silica and water are the same, they have a similar tetrahedrally symmetric concordant geometry, and they both show an unusual tendency to become less dense at a given temperature, but more liquid when pressurized.

The macroscopic difference between water and silicon originated from the combination of the microscopic world. It is hoped that this principle will be extended to other substances, such as liquid carbon and silicon, which are structurally similar to water and silicon. The ultimate goal is to develop a general theory of how glass-formers differ from crystal-formers, which scientists have so far been unable to do.