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Cubic Close Packing


There are three types of cubic lattices corresponding to three types of cubic close packing, as summarized in the following table. Now that the Kepler conjecture has been established, hexagonal close packing and face-centered cubic close packing, both of which have packing density of eta=pi/(3sqrt(2))=0.74048..., are known to be the densest possible packings of equal spheres.

lattice typebasis vectorspacking density
simple cubic (SC)x^^, y^^, z^^pi/6 approx 52.3%
face-centered cubic (FCC)1/2(y^^+z^^), 1/2(x^^+z^^), 1/2(x^^+y^^)pi/(3sqrt(2)) approx 74.0%
body-centered cubic (BCC)1/2(-x^^+y^^+z^^), 1/2(x^^-y^^+z^^), 1/2(x^^+y^^-z^^)pisqrt(3)/8 approx 68.0%

Simple cubic packing consists of placing spheres centered on integer coordinates in Cartesian space.

FaceCenteredCubicClosePackingLayers

Arranging layers of close-packed spheres such that the spheres of every third layer overlay one another gives face-centered cubic packing. To see where the name comes from, consider packing six spheres together in the shape of an equilateral triangle and place another sphere on top to create a triangular pyramid. Now create another such grouping of seven spheres and place the two pyramids together facing in opposite directions.

FaceCenteredCubicClosePackingCubeStellaOctangula

Connecting the centers of eight of the spheres, a cube emerges (Steinhaus 1999, pp. 203-204) in which the centers of the other six spheres lie at the centers of the faces of the cube. Connecting the centers of these 14 spheres gives a stella octangula, illustrated above.

FaceCenteredClosePackingDiagramCubicFCCUnitCell

Consider the cube defined by 14 spheres in face-centered cubic packing. This "unit cell," one face of which is illustrated above in schematic form, contains eight 1/8-spheres (one at each polygon vertex) and six hemispheres. The total volume of spheres in the unit cell is therefore

V_(FCC spheres)=(8·1/8+6·1/2)(4pi)/3r^3
(1)
=(16)/3pir^3.
(2)

The diagonal of a face of the unit cell is 4r, so each side is of length 2sqrt(2)r. The volume of the unit cell is therefore

 V_(FCC unit cell)=(2sqrt(2)r)^3=16sqrt(2)r^3,
(3)

giving a packing density eta=V_(spheres)/V_(cell) of

eta_(FCC)=pi/(3sqrt(2))
(4)
=0.74048...
(5)

(OEIS A093825; Conway and Sloane 1993, p. 2).

CubicClosePackingClusterCubicClosePackingCuboct

In face-centered cubic packing, each sphere is surrounded by 12 other spheres. Taking a collection of 13 such spheres gives the cluster illustrated above. Connecting the centers of the external 12 spheres gives a cuboctahedron (Steinhaus 1999, pp. 203-205; Wells 1986, p. 237).

CubicBCCUnitCell

In cubic body-centered packing, each sphere is surrounded by eight other spheres. The figure above shoes the unit cell in body-centered cubic packing. In this configuration, a single full sphere occupies the center and is surrounded by eight 1/8-spheres. The total volume of spheres in the unit cell is therefore

V_(BCC spheres)=(8·1/8+1)(4pi)/3r^3
(6)
=8/3pir^3.
(7)

The space diagonal of the unit cell is 4r, so each side is of length 4r/sqrt(3). The volume of the unit cell is therefore

 V_(BCC unit cell)=((4r)/(sqrt(3)))^3=(64)/(3sqrt(3))r^3,
(8)

giving a packing density eta=V_(spheres)/V_(cell) of

eta_(BCC)=(sqrt(3)pi)/8
(9)
=0.680174...
(10)

(OEIS A268508).

SquashedCubicSquashedHexagonal

If spheres packed in a cubic lattice, face-centered cubic lattice, and hexagonal lattice are allowed to expand uniformly until running into each other, they form cubes, hexagonal prisms, and rhombic dodecahedra, respectively. In particular, if the spheres of face-centered cubic packing are expanded until they fill up the gaps, they form a solid rhombic dodecahedron (left figure above), and if the spheres of hexagonal close packing are expanded, they form a second irregular dodecahedron consisting of six rhombi and six trapezoids (right figure above; Steinhaus 1999, p. 206) known as the trapezo-rhombic dodecahedron. The latter can be obtained from the former by slicing in half and rotating the two halves 60 degrees with respect to each other. The lengths of the short and long edges of the rotated dodecahedron have lengths 2/3 and 4/3 times the length of the rhombic faces. Both the rhombic dodecahedron and trapezo-rhombic dodecahedron are space-filling polyhedra.


See also

Circle Packing, Cube, Cubic Lattice, Hexagonal Close Packing, Kepler Conjecture, Kepler Problem, Sphere Packing

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References

Conway, J. H. and Sloane, N. J. A. Sphere Packings, Lattices, and Groups, 2nd ed. New York: Springer-Verlag, 1993.Sloane, N. J. A. Sequences A093825 and A268508 in "The On-Line Encyclopedia of Integer Sequences."Steinhaus, H. Mathematical Snapshots, 3rd ed. New York: Dover, pp. 202-203, 1999.Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, p. 29, 1986.

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Cubic Close Packing

Cite this as:

Weisstein, Eric W. "Cubic Close Packing." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CubicClosePacking.html

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