In network forming liquid and glasses, structural order extends well beyond nearest-neighbor distances, leading to the appearance of a first sharp diffraction peak in the total neutron structure factor. Whenever available, partial structure factors convey an even more detailed set of information on the structure of these systems. In particular, the existence of concentration fluctuations at short and intermediate range distances is revealed by the behavior of the corresponding concentration-concentration structure factor. Computer modelling at the atomic scale is ideally suited to link the presence of this signature to specific features in real space, provided chemical bonding is accurately described. We reach this goal by addressing these issues within the framework of first-principles molecular dynamics.
This talk will focus on short and intermediate range order in prototypical disorder AX₂ networks: SiSe₂, GeSe₂ and GeSe₄. We have obtained a picture of amorphous SiSe₂ fully consistent with diffraction data, the number of Si atoms in corner and edge-sharing configurations following closely the experimental estimate. By focusing on all possible sequences of three Si atoms, we show that the series made of both corner and edge-sharing connections are more numerous than those formed exclusively by edge-shared atoms.
In the case of liquid GeSe₄, we have focused on a comparative study of the charge-charge S_zz and the concentration-concentration S_cc structure factors. We found that the first sharp diffraction peak is present in S_cc while it is absent in S_zz. This shows the inadequacy of point-like charge models in the treatment of highly ionic disordered materials, the two structure factors being simply proportional witin this approach. Our results suggest that a structural frustration due to the constraint of charge neutrality lead to the appearance of concentration fluctuations at intermediate range distances.

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