Close-packed structures

Lattice is close-packed, if its packing factor is greater than that of random close packing, 0.87, and is loosely packed if its packing factor is less than that of random loose packing, 0.75.

Ideally packed (δ=1) structures can be represented by the stacking sequence of close-packed layers:

stacking		elementary metals		selected alloys
AB	h	D_6h⁴	A3 hcp	XY	C_6v⁴	B4 wurtzite
ABC	c	O_h⁵	A1 fcc	XY	T_d²	B3 zincblende
ABAC	ch	D_6h⁴	A3' α-La	XY	D_6h⁴	B8₁
ABCBC	cchhc	D_3d³		XYXXY	D_3d³	D5₁₉ Al₃Ni₂
ABABCBCAC	chh	D_3d⁵	α-Sm	XYY	D_3d⁵	C19

Not ideally packed structures can be obtained by deformations and partitions of the above lattices, here are some examples:

A2 bcc – rhombohedral deformations of fcc (δ=.92)
A10 & Ai, A7 – other rhombohedral deformations of fcc (no elementary crystals with high δ)
A6 & Aa – tetragonal deformations of fcc (In has δ=.94, α-Pa has δ=.93)
B2 – bipartition of bcc (AlNi)
Bh with c/a≈1.6 – bipartition of hcp (no example)
L1₀ – bipartition of Aa (CuAu)
L1₂ – 1:3 partition of fcc (AlNi₃)
3:5 partition of fcc (Al₃Ni₅)

For metal alloys with unequal size ions the diversity of close-packed structures grows. For the case, when the smaller ions fit into the voids of the close-packed lattice formed by the larger ones, see ionic compounds with close-packed sublattice.

ABC notations (see Wikipedia):