The RMC model for SiO2, as published in Nature, was not a perfect 4-connected 3D net, contrarily to the RDM model based on the carnegieite, starting from the atomic coordinates given by Barth for its early description of high-cristobalite.
Fitting both neutron and
X-ray diffraction data by RMC based on the RDM model, extended to 1536
atoms, was straightforward.
Constraints for maintaining the initial connectivity scheme produced a model for glassy SiO2 with exclusively 6-membered rings, quite different from the "classic" RMC approach, leading to the occurence of 4-, 5-, 6-, 7-, and 8-membered rings.
Observed (+++) and calculated (¾¾
) interference functions corresponding to the RMC modelling of glassy SiO2,
starting from the a-carnegieite RDM model.The
difference functions are in the lower parts.
a) Neutrons, Rp = 1.48 % ;
b) X-ray,Rp = 2.00 %.
Projection of the RMC model for glassy SiO2, starting from the a-carnegieite RDM model. The distorted hexagonal tunnels are finger print of the starting high-cristobalite-like model :