Crystal Lattices and Unit Cells

Introduction

Crystal Lattices and Unit Cells

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. The entire lattice is generated by the repetition of the unit cell in different directions.

 

Parameters of a Unit Cell

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.

 

Types of Unit Cell

Types of Unit Cell

 

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

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

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