hexagonal close-packed
/ hek-SAG-uh-nuhl klohs-PAKT /
Lay marbles flat on a table and push them together: each marble settles into a snug ring of six, making a honeycomb of triangles — a hexagonal layer. Now build upward, dropping the second layer into the dimples of the first. When the third layer sits directly above the first, repeating ABAB forever, you have hexagonal close-packing.
Hexagonal close-packed, or hcp, is one of the two ways to stack close-packed layers as tightly as spheres allow. Each layer is a hexagonal sheet of touching atoms, and successive layers alternate in just two positions, an ABAB pattern. Like all close packings, every atom has twelve nearest neighbours and the spheres fill about 74 percent of space. Its unit cell is a hexagonal prism, and the structure carries a built-in axis — the stacking direction — that makes it less symmetric than a cube, so hcp metals often behave differently along that axis than across it.
Hcp matters because important metals adopt it: magnesium, zinc, titanium, cobalt, and the everyday form of beryllium and zirconium. Its lower symmetry, with that special stacking axis, gives these metals direction-dependent properties and often makes them harder to deform than fcc metals. The key thing not to miss is that hcp and fcc are equally dense — both are close-packed at about 74 percent — and differ only in their layer sequence: ABAB for hcp, ABCABC for fcc.
Titanium, prized for being strong yet light in aircraft and implants, is hcp at room temperature. Its stacking-axis structure is part of why titanium parts must be worked and machined more carefully than aluminium ones.
Strong, light titanium is hcp — its axis makes it trickier to shape.
Hcp is a structure, not a Bravais lattice on its own — its true description is a simple hexagonal lattice with a two-atom basis. The two close-packed layers per repeat come from that basis, not from two different lattices.