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.