Figure 7 The coordinate axes , , are aligned along directions of the first-nearest atoms to the atom at the origin A. The coordinate axes 1, 2, 3 are along directions of the second-nearest atoms to the origin at A. The corresponding six atoms form the hexagon (indicated by dashed lines) of interatomic spacing = 31/2a0, where a0 is the interatomic distance between the first-nearest neighbors.