the hexagonal close-packed structure

The hexagonal close-packed structure is the other way to stack close-packed layers. Where FCC repeats ABCABC, HCP repeats ABAB: the third layer sits directly above the first. If close packing is a pyramid of oranges, HCP and FCC are simply two different rhythms of laying down the layers, equally dense but with different long-range patterns.

There is a subtlety worth stating plainly: HCP is not itself a Bravais lattice. It is a simple hexagonal lattice decorated with a two-atom motif (one atom at 0,0,0 and one at 2/3,1/3,1/2). The conventional cell contains 2 atoms; along the vertical c axis the ideal ratio of height to width is c/a = sqrt(8/3) = 1.633 when the spheres are perfect and touching. Like FCC it gives coordination number 12 and packing factor 0.74. Common HCP metals are magnesium, zinc, titanium, cobalt, cadmium, and beryllium.

Because HCP has far fewer easy slip systems than FCC (slip is dominated by the single basal plane), many HCP metals are harder to deform at room temperature and can be brittle or strongly anisotropic. Real metals rarely hit the ideal c/a exactly: zinc and cadmium are stretched (c/a about 1.86), titanium and magnesium are slightly squashed (c/a about 1.59), and that deviation changes which slip systems switch on.