Graphite, Bragg reflections

Elastic neutron diffraction was first performed (analyzer in Fig. 1 set to zero energy transfer) to establish the structure of the monolayer at low temperature. Three Bragg reflections were observed which could be indexed by a triangular lattice having a nearest-neighbor distance about 10% smaller than required for a 3 X /3 R30° commensurate structure (every third carbon hexagon in the graphite basal plane occupied). 

Molecular orientational order in adsorbed monolayers can be inferred indirectly from elastic neutron diffraction experiments if it results in a structural phase transition which alters the translational symmetry of the 2D lattice. In such cases, Bragg reflections appear which are not present in the orientationally disordered state. Experiments of this type have inferred orientational order in monolayers of oxygen (41) and nitrogen (42) adsorbed on graphite. However, these experiments have not observed a sufficient number of Bragg reflections to determine the molecular orientation by comparing relative Bragg peak intensities with a model structure factor. 

Figure 3.9 Schematic diagram of the 10-m small-angle neutron scattering faciiity at Oak Ridge, TN. The neutron beam used by the instrument is transported from the beam room to the SANS facility level by Bragg reflection from pyrolitic graphite crystals scattering at 90°. Courtesy of Dr. G. D. Wignall.

Hirai et al. [4], in research first reported in 1991, catalogued similar results from studies of shock-compressed graphite using a rapid cooling technique. They observed carbon diamond polytype reflections in their heated, shock-compressed carbon samples, but in addition they recorded the (200), (222) and (420) symmetry forbidden Bragg reflections of cubic diamond in their diffraction analysis. 

Figure 1. X-ray patterns of calcined MSU-1 and -4 Silica obtained with Tergitol 15S12 and Tween 20 as templating agents. The patterns were recorded with a Bruker D5000 diffractometer in Bragg-Brentano reflection geometry. Cu-L radiation was employed that was monochromatized by a graphite single crystal in the diffracted beam.

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