Breaking local symmetry: Why water freezes but silica forms a glass

Researchers have simulated water and silica at low temperature. Despite structural similarities, the two liquids act differently when they are cooled: water freezes into ice, while silica continues to supercool, and eventually forms a glass. This arises from poor symmetry-breaking in silica; although atoms arrange properly in the first shell in both liquids, local rotational symmetry is harder to break in the second shell in silica, because of the less directional Si-O bonds.