First Sharp Diffraction Peak

Figure 8 Behavior of the first sharp diffraction peak of PB with experimental scattering lengths for carbon and deuterium. The deuterium atoms are placed at their mechanical equilibrium positions determined by the positions of the united atom centers and the equilibrium CH bond length and HCH and HCC bond angles along a united atom MD trajectory. With increasing pressure (values given in the legend, simulation performed at T = 293 K), the first sharp diffraction peak shifts to larger q as expected but unexpectedly decreases in height.

Neutrons have been used to study glasses and show a first sharp diffraction peak at low angles. This implies that there is some ordering in the glass. Intentional patterns of voids in glasses give rise to such peaks. Table 10.9 provides a comparison of the parameters for XRD and neutron diffraction. 

As an example of regularity in the diffraction patterns, the waxs spectra for different polycarbonates of bisphenol A (54) are shown in Figure 12. The figure shows different diffraction patterns starting from the monomer (x = 1, which is crystalline), the pentamer (x = 5), and other oligomers up to the bulk. It is interesting to note that there is a correlation of the most intense reflection in the crystalline monomer with the amorphous halo in the BPA polymers and that the first sharp diffraction peak gets broader as one moves from the pentamer to the polymer. 

The first sharp diffraction peak (FSDP) in Figure l-l(a), the position of peaks in the RDF(r) curve which correspond to the interatomic distances, andpeak-to-peak distances in the C(r) curve, which correspond to the correlation periodicity or the size of the repeated structinal unit, are important parameters in order to discuss the MRO of the xerogel and glasses. 

Elliott S.R. Origin of the first sharp diffraction peak in the structure factor of covalent glasses. Phys. Rev. Lett. 1991 67 711-714... 

Fig. 5.6 Correlation between the calculated Young s modulus (E) and the correlation length (L) of the first sharp diffraction peak of alkali silicate glasses. The plots is reproduced by the data values reported in [56]...

J. Du, L.R. Corrales, Compositional dependence of the first sharp diffraction peaks in alkali sihcate glasses a molecular dynamics study. J. Non-Cryst. Solids 352,3255-3269 (2006)... 

As an example. Fig. 6 shows two of the 5ap( ) (more details are given elsewhere [40, 56]). Note that the first sharp diffraction peak (at q=. l A ) in NSjc is mostly due to Si-O and 0-0 correlations, and thus in the Si-Si correlations it shows up only as a mild shoulder, unlike Si02 where there is a clear peak. This already reflects that the rigid network of SiO " tetrahedra present in pure Si02 to some extent is broken up in NSx, due to the presence of the network modifier. But the prepeak at = 0.95 A now can be very clearly recognized. 

Wilson, M. Madden, P. A. (1998) Voids, Layers, and the First Sharp Diffraction Peak in ZnCl2, Phys. Reo. Lett. Vol. 80 532-535, in press. 

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