Definition of Unit Cell: Unit cell is the smallest repeating structural unit of a crystalline solid (or crystal lattice) which when repeated in different directions produces the crystalline solid (lattice).
When the lattice points of a space lattice, are joined by straight lines a large number of parallelepiped are formed. Each parallelepiped is the smallest and a complete unit. When these units are repeated in space, a crystal lattice is obtained. This three dimensional smallest group of lattice points is known as unit cell. Unit cells when repeated in space, a crystal is obtained. The concept of unit cell will become more. Clear if we consider a block made of equal sized bricks. If so, then each brick is a unit cell of this block.
Generally we can define a unit cell as, it is the smallest, but complete unit in space lattice which when repeated in three dimensions generates the crystal. A unit cell is the smallest building block of a crystal.
A unit cell is characterized by the following parameters:
- Edges or Edge lengths: The intersection of two faces of crystal lattice is called as edge. The three edges denoted by a, b and c represents the dimension (lengths) of the unit cell along three axes. These edges may or may not be mutually perpendicular.
- Angles between the edges (or planes): There are three angles between the edges of the unit cell represented as alpha, beta and gamma.
- The angle alpha is between edges b and c.
- The angle beta is between edges a and c.
- The angle gamma is between edges a and b. The cell is defined with the help of these parameters of its unit cell.
The distances a, b and c in a unit cell are called the unit lengths and the angles are called as axial angles. It must be remembered that the chice of unit cell is somewhat arbitrary, because the lattice points can be joined in more than one different ways. However, a unit cell is so chosen that it should exhibit the external geometry of the crystals.
- unit cell
- unit cell definition
- lattice structure
- crystalline structure
- ionic lattice
- space lattice