Evidence of concentration fluctuations in disordered network-forming systems: the case of GeSe4 and SiSe2

In a search for the physical origin of the first sharp diffraction peak (FSDP) in the concentration-concentration partial structure factor SCC (k) of disordered network-forming materials, we perform first-principles molecular dynamics simulations of liquid GeSe4 (l-GeSe4) and liquid SiSe2 (l-SiSe2). These systems are designed to provide clues on the relationship between the appearance of an FSDP in the SCC(k) structure factor and the degree of chemical order. Short-range chemical order is more pronounced in l-GeSe4 and in l-SiSe2 than in liquid GeSe2. For the latter system, our level of theory does not reproduce the FSDP in the experimentally observed SCC (k) structure factor. We find that a distinct FSDP shows up in the partial structure factor SCC (k) for l-GeSe4. In SCC (k) for l-SiSe2, we also find a feature at the FSDP location, although it is smaller compared to GeSe4. Given the tight correlation existing between chemical order and ionicity, these results suggest that the ionic character of the bonds plays a crucial role in inducing concentration fluctuations at intermediate-range distances