Chemistry Reference
In-Depth Information
structure, spheres are arranged in a single close-packed layer to form a
basal plane with each atom surrounded by six others. The next layer is
added by placing spheres in alternating threefold hollows of the basal plane.
If a third layer is added such that the spheres are directly above the spheres
in the basal plane, we obtain the hcp structure. (If the third layer atoms are
added in the hollows
not
directly above the basal plane, the fcc structure
is obtained.)
Hexagonal close-packed (hcp) materials have a sixfold symmetry axis
normal to the basal plane. Using a three-axis system to define Miller indices
for this structure is unsatisfactory, as is demonstrated in Fig. 4.9. The two
planes highlighted in Fig. 4.9 are equivalent by symmetry, and yet their
Miller indices do not show this relationship. This is unfortunate since one of
the reasons Miller indices are useful is that equivalent planes have similar
Figure 4.10
Examples of Miller indices for hcp materials using the four axis, four index
system.
