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. Upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest neighbor atoms. Not all solids are single crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals (called “grains“) is a true crystal with a periodic arrangement of atoms. Still, the whole polycrystal does not have a periodic arrangement of atoms because the periodic pattern is broken at the grain boundaries.
Polymorphism and Allotropy
Some crystalline materials may have more than one crystal structure, a phenomenon known as polymorphism. Polymorphism is the occurrence of multiple crystalline forms of a material. According to Gibbs’ rules of phase equilibria, these unique crystalline phases depend on intensive variables such as pressure and temperature. Polymorphism is related to allotropy, which refers to chemical elements. That means polymorphism is the more general term for any crystalline material, including alloys and chemical elements. Each polymorph is a different thermodynamic solid state, and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in the industrial manufacture of crystalline products.
One familiar example is found in carbon. Graphite (a soft, black, flaky solid, a moderate electrical conductor) is the stable polymorph under ambient conditions, whereas diamond (an extremely hard, transparent crystal with carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.) is formed at extremely high pressures.
Also, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at 913 °C. Zirconium is HCP (alpha) up to 863°C, where it transforms to the BCC (beta, zirconium) form. The problem is that these phase transitions make this material very brittle.