The diffraction of electrons by graphite

Abstract

The structure first attributed to graphite by Debye and Scherrer, and by Hull from X-ray powder patterns was confirmed by Hassell and Mark, by Bernal, and, finally, by Mauguin and Ott who relied mainly on single crystal X-ray photographs. The assigned structure is such that the hexagonal unit cell has the dimensions, a = 2·46 and c = 6·79 A, and contains two pairs of crystallographically different carbon atoms of coordinates (0, 0, 0), (0, 0, ½), (⅓, ⅔, u ), and (⅔, ⅓, u + ½), where, according to Ott, u < 0·004. Hence we may picture the graphite crystal as consisting of carbon atoms situated at the points of plane, or nearly plane hexagonal network sheets parallel to, and equidistant from, each other, and so superimposed that the projections from hexagon centres in one sheet pass through atoms in the two neighbouring sheets. Thus the atoms in alternate sheets are superimposed. Trendelenburg, Miwa, and Jenkins studied the diffraction of electrons by graphite powder and concluded that their results were in agreement with X-rays, apart from the absence or undue weakness of the 001 diffractions. This difference was satisfactorily explained by these workers as being due to the effect of the flake-like form of the crystals and the inner potential of graphite, the latter being known from measurements by Yamaguti and Jenkins of electron-diffraction patterns obtained by reflexion at single-crystal cleavage faces. A similar conclusion was reached by other workers who, however, have not published diffraction patterns in substantiation of their findings. Finally, Aminoff and Broome considered that the spot patterns and a Kikuchi line pat-tern obtained by them by transmission of electrons through relatively thin and thicker graphite flakes respectively were in general agreement with the accepted graphite structure, and indicated hexagonal symmetry of the structure with respect to the c axis or cleavage face normal. In the course of a study of the lubricating properties of graphite, we obtained transmission patterns of graphite films formed by the drying-out of a colloidal artificial graphite (“Aquadag”) solution. Although these patterns at first sight appeared to resemble those hitherto obtained by X-ray and electron diffraction, a closer inspection revealed the existence of several rings which had previously not been observed, doubtless because of insufficient resolution, and which did not appear to fit in with the current view of the structure of the graphite lattice. It is the object of this communication to set forth these differences and to elucidate their origin.