Hexagonal packing Bravais cell

Figure 6. TEM images of STAC-1 viewed down the a axis of a hexagonal unit cell (indicated by [M/]h) or the [110] direction of a cubic unit cell (indicated by [M/]c). The crystal is dominated by ABCABC close packing (indicated on (a)) with one stacking fault (marked by a horizontal line). A Fourier transform optical diffraction pattern with both Miller-Bravais indices to the hexagonal unit cell and Miller indices (in parentheses) to the cubic unit cell is inserted in (b). Simulated images based on a proposed model (right) are also inserted with specimen thickness of 30 nm, and lens focuses of—30 nm (a) and —10 nm (b).

The next degree of complexity is encountered when two or more atoms of the same kind are associated with each point of a Bravais lattice, as exemplified by the hexagonal close-packed (HCP) structure common to many metals. This structure is simple hexagonal and is illustrated in Fig. 2-15. There are two atoms per unit cell, as shown in (a), one at 0 0 0 and the other at f (or at i, which is an equivalent position). Figure 2-15(b) shows the same structure with the origin of the unit cell shifted so that the point 1 0 0 in the new cell is midway between... 

Graphene is a one-atom-thick planar layer of graphite, where the atoms are packed in a hexagonal ( honeycomb ) crystal lattice. The carbon atoms are sp -bonded with a bond length of 1.42 A. The crystal lattice has two atoms per unit cell, A and B, and it is rotationally symmetric for rotations of 120° around any lattice point. One can view the honeycomb lattice as a triangular Bravais lattice with a basis of two atoms per unit cell. Fig. 2a. 

The third important crystal structure of metals is the hexagonal close-packed structure, abbreviated hep. This structure is not a Bravais lattice as not all atoms occupy identical positions. Looking at figure 1.6, it can be seen that the atom at the front right edge of the cell has a neighbour that can be... 

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