Distinct signature of local tetrahedral ordering in the scattering function of covalent liquids and glasses.

Tetrahedral amorphous materials such as SiO2, GeO2, Si, Ge, C, and chalcogenides are extremely important in nature and technology. It is known that covalent bonding favors local tetrahedral order in these materials. However, how to extract information on this structural order from the scattering function has remained elusive. By analyzing the structure of simulated SiO2 and experimental data of various tetrahedral materials, we show that the lowest wave number peak, known as the first sharp diffraction peak (FSDP), and a few higher wave number ones in the scattering functions come from the characteristic density waves of a single tetrahedral unit. FSDP is thus a direct measure of the tetrahedrality. This finding opens the door for long-awaited experimental access to the characterization of disordered amorphous structures.