Crystal Lattices and Unit Cells, End Centred, Formation of Lattice Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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 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 3-D 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.

Characteristic of Unit Cell

The following characteristics define a unit cell:

• A unit cell has three edges a, b and c and three angles , and between the respective edges
• The a, b and c may or may not be mutually perpendicular
• The angle between edge b and c is , a and c are β and that of between a and b is

Unit cells are of two types namely:

• Primitive Unit Cells
• Centred Unit Cells

Parameters of a Unit Cell

Unit Cell Parameters

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.

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

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