Crystal Lattices and Unit Cells
Crystalline solids have long-range order and the closely packed constituent particles are arranged in an ordered three dimensional pattern. The structure of the crystalline solids can be represented as an ordered three dimensional arrangement of points. Here each point represents the location of a constituent particle and is known as lattice point and such an arrangement is called a crystal lattice.
In a crystal lattice you can select a group of points which can be used to generate the whole lattice. Such a group is called repeat unit or the unit cell of the crystal lattice. The unit cell is characterized by three distances along the three edges of the lattice and the angles be-tween them.
On the basis of the external appearance the known crystals can be classified into seven types. These are called crystal systems. In terms of the internal structure also the crystal lattices contain only seven types of unit cells.
- Cubic
- Tetragonal
- Orthorhombic
- Rhombohedral
- Hexagonal
- Monoclinic
- Triclinic
A unit cell containing a lattice point each at the centers of its faces in addition to the lattice points at the corners is called a face centered (F) unit cell. On the other hand a unit cell with lattice points at the center of the unit cell and at the corners is called a body centered unit cell (I).
The seven crystal systems when combined with these possibilities give rise to 14 lattice types. These are called Bravais lattices.
