First Sharp Diffraction Peak Anomalies

We examine now structural anomalies that correlate to those detected previously, and follow the total neutron weighted structure factor SN(k) of glassy NS2 with pressure. In Fig. 11.22 is represented the total computed structure factor SN(k), obtained from a linear combination of the partial structure factors Sij (fe), defined by  

The most revealing partial structure factor (Sisi-o) is represented for the three selected pressures in the inset of Fig. 11.22. For all (SN(k), Sy(k)), aLorentzian fit is used in order to extract the position kpsop and width Akpsop- An anomalous behavior in the FSDP for some of the partial structure factors is found (Fig. 11.23), as the position kpsDp maximizes for the partials involving the network-forming species 

The position of the FSDP usually reflects some repetitive characteristic distance between structural units [130]. The present findings of Fig. 11.23, thus, indicates that in the same pressure interval where BB constraints soften to accommodate the increase of stress due to the connectivity increase, a typical length scale of distance L = l.lIkpsDP (4.05A for the 0-0 correlations ) emerges, and then decreases. Similarly, the broadening of the FSDP is indicative of a characteristic correlation length emerging in the same pressure window. This follows from the well-known 

Such correlations between anomalies in dynamic properties and anomalies in the FSDP parameters have a one to one correspondence with results [51] obtained from a systematic study on amorphous As-Se where rigidity is tuned by composition. Here all FSDP anomalies are also located in or close to an intermediate phase reported experimentally [40]. This underscores, therefore, not only the generic behavior of such trends but also the fact that there is clearly a structural signature for the intermediate phase, in contrast with previous statements based solely [50] on the analysis of the total structure factor. As seen from Fig. 11.23 and from [51], FSDP parameters of the total neutron or X-ray SN(k) may not necessarily exhibit such a structural signature. 

In this chapter, we have shown that structural information obtained from molecular dynamics is able to provide a rigorous atomic-scale basis to phenomenological constraint counting concepts applied to network glasses. An initial step is the production of realistic models of glasses whose structure has to agree with what is found from experiments. In addition, the dynamic properties of melts and liquids can be compared, whenever possible, to viscosity or diffusivity measurements. 

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