# Crystal Lattices and Unit Cells

The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements. The periodic structure of an ideal crystal is most easily described by a lattice. The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure.

## Crystal Lattice

The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space. ### Characteristics of Crystal Lattice

• In a crystal lattice, each atom, molecule or ions (constituent particle) is represented by a single point.
• These points are called lattice site or lattice point.
• Lattice sites or points are together joined by a straight line in a crystal lattice.
• When we connect these straight lines we can get a three-dimensional view of the structure. This 3D arrangement is called Crystal Lattice also known as Bravais Lattices.

## Unit Cell

Unit Cell is the smallest part (portion) of a crystal lattice. It is the simplest repeating unit in a crystal structure. The entire lattice is generated by the repetition of the unit cell in different directions.

### 1. Parameters of a Unit Cell There are six parameters of a unit cell. These are the 3 edges which are a, b, c and the angles between the edges which are α, β, γ. The edges of a unit cell may be or may not be perpendicular to each other.

### 2. Types of Unit Cell #### 1. Primitive Unit Cells

When the constituent particles occupy only the corner positions, it is known as Primitive Unit Cells.

#### 2. Centred Unit Cells

When the constituent particles occupy other positions in addition to those at corners, it is known as Centred Unit Cell. There are 3 types of Centered Unit Cells:

• Body Centred: When the constituent particle at the centre of the body, it is known as Body Centred Unit cell.
• Face Centred: When the constituent particle present at the centre of each face, it is known as Face Centered Unit cell.
• End Centred: When the constituent particle present at the centre of two opposite faces, it is known as an End Centered Unit cell.