Beyond short-range order length scales in disordered materials: first principles modeling of SiSe2

First-principles molecular dynamics simulations are carried out to study the structural properties of liquid SiSe2. This system is a prototype of disordered network-forming material characterized by intermediate range order, i.e. structural order involving distances well beyond nearest neighbors. Our simulations show that the atomic structure is chemically ordered, with most of the Si atoms fourfold coordinated in SiSe4 tetrahedra. The resulting population of Si sites, established in terms of connectivity among tetrahedra, is in good agreement with experimental predictions.