The spatial arrangement of atoms within a material plays an important role in determining the properties of the material.
Solids can be classified into three major types
An amorphous solid does not have a well-defined structure; it is formless. There is no recognizable long-range order in the positioning of atoms within the material. The atomic arrangement in any given section of an amorphous material will look different from the atomic arrangement in any other section of the material (see Fig. 1(a)).
In a polycrystalline solid there are many small regions each having an well-organized structure but differing from its neighboring regions and separated by grain boundaries (see Fig. 1(b)).
In a crystalline solid atoms are arranged in an orderly array that defines a periodic structure called the lattice. A unit cell when repeated by itself produces a crystalline solid. A unit cell contains complete information regarding the arrangement of atoms, and hence the unit cell can be used to describe the crystal structure. Simply stated, a unit cell is a small portion of any given crystal that could be used to reproduce the crystal. A primitive cell is the smallest unit cell possible (see Fig. 1(c)).
Fig. 1 General classification of solids based on the degree of atomic order: (a) amorphous, (b) polycrystalline, and (c) crystalline.