Arrangements for stacking spheres, each one characterized by a packing efficiency (percentage of total volume occupied by spheres)
1) Simple cubic unit cell
8 atoms define corners – atoms touch along edges, but not along diagonal
Coordination # = 6. 4 in layer, 1 above, 1 below
1 atom/unit cell (1/8 ´ 8 atoms)
2) Body-centered cubic unit cell
8 atoms define corners plus one in center – atoms touch central atom, not each other
Coordination # = 8. 4 above and 4 below.
2 atoms/unit cell (1/8 ´ 8 atoms + 1 atom in center)
3) Face-centered cubic unit cell
8 atoms define corners plus 6 in center of each face – Corner atoms touch face atom, not each other
Coordination # = 12. 4 in layer, 1 above, 1 below, 6 faces
4 atoms/unit cell (1/8 ´ 8 crnr atoms + ½ ´ 6 face atoms)
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