Entry by Emily Redston, AP 225, Fall 2011
A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances; that is, long-range order exists such that, upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern. If this long-range atomic order is absent, an amorphous material will form. The crystal structure of a material is the manner in which atoms, ions, or molecules are spatially arranged. There is an extremely large number of different crystal structures all having long-range order; these vary from relatively simple structures for metals to exceedingly complex ones, as displayed by some of the ceramic and polymeric materials. When describing crystalline structures, atoms are thought of as being solid spheres having well-defined diameters. This is an atomic [[hard sphere] model in which spheres representing nearest-neighbors atoms touch one another. The term lattice is oftentimes used to describe a three-dimensional array of points coinciding with atom positions.
The atomic order in crystallline order in crystalline solids indicates that small groups of atoms form a repetitive pattern. Thus, in describing crystal structures, it is often convenient to sub-divide the structure in small repeat entities called unit cells. A unit cell is choisen to represent the symmetry of the crystal structure, wherein all the atom positions in the crystal may be generated by translation of the unit cell integral distances along each of its edges. Thus, the unit cell is the basis structural unit of the crystal structure and defines the crystal the crystal structure by virtue of its geometry and the atom positions within.
Briefly, since they are relevant to hard sphere colloidal packing, I will go over two essential structures: face-centered cubic and hexagonal close packed. For hard-sphere models, you typically end up with relatively large numbers of nearest neighbors and dense atomic packings due to the minimal restrictions as to the number and position of nearest-neighbor atoms.