Charge fluctuations and concentration fluctuations at intermediate-range distances in the disordered network-forming materials SiO2, SiSe2, and GeSe2

We calculate the concentration-concentration partial structure factor S-CC(k) and the charge-charge structure factor S-zz(k) of liquid SiO2, amorphous SiSe2 and liquid GeSe2 using first-principles molecular dynamics. These systems are characterized by the occurrence of intermediate range order, as evidenced by a first sharp diffraction peak (FSDP) at low k values in the total neutron structure factor. We show that a FSDP in the concentration-concentration partial structure factor S-CC(k) is generally associated with a small departure from chemical order. This feature tends to vanish either when sufficiently high levels of structural disorder set in, or, oppositely, when the chemical order is essentially perfect. For none of these networks, a FSDP is observed in the charge-charge structure factor S-zz(k), i.e., fluctuations of charge do not occur over intermediate range distances. The constraint of charge neutrality is at the very origin of the appearance of fluctuations of concentration. These are observed when the atoms occur in configurations with different coordinations.