Question

discuss different types of unit cell in crystal​

Answer 1

Answer:

SOLID STATE

Crystal Lattices and Unit Cells

Crystal lattice is the depiction of three dimensional arrangements of constituent particles (atoms, molecules, ions) of crystalline solids as points. Or the geometric arrangement of constituent particles of crystalline solids as point in space is called crystal lattice.

crystal lattice solid state

Characteristics of crystal lattice:

Each constituent particle is represented by one point in a crystal lattice.

These points are known as lattice point or lattice site.

Lattice points in a crystal lattice are joined together by straight lines.

By joining the lattice points with straight lines the geometry of the crystal lattice is formed.

Unit Cell: The smallest portion of a crystal lattice is called Unit Cell. By repeating in different directions unit cell generates the entire lattice.

parameter of unit cell

Parameters of a unit cell:

A unit cell is characterized by six parameters. These parameters are three edges (a, b and c) and angles between them (α, β and γ).

Dimensions along the edges of a unit cell is represented by a, b and c.

Edges of unit cell may or may not be mutually perpendicular.

The angle between b and c is represented by α, between a and c by β and between a and b by γ.

Types of Unit Cell: There are two types of unit cells – Primitive and Centred Unit Cells.

Primitive Unit Cells: When particles in unit cell are present only at the corners, it is called the primitive unit cell.

Centred Unit Cells: When particles are present at other positions in addition to those at corners in a unit cell, it is called a Centred Unit Cell.

There are three types of Centred Unit Cell

(a) Body Centred Unit Cells: If one constituent particle lies at the centre of the body of a unit cell in addition to the particles lying at the corners, it is called Body-Centred Unit Cell.

(b) Face-Centred Unit Cells: If one constituent particle lies at the centre of each face besides the particles lying at the corner, it is known as Face-Centred Unit Cells.

(c) End-Centred Unit Cell: If one constituent particle lies at the centre of any two opposite faces besides the particles lying at the corners, it is known as End-Centred Unit Cell. It is also known as base-centred unit cell.

There are seven types of unit cell formed. These are Cubic, Tetragonal, Orthorhombic, Monoclinic, Hexagonal, Rhombohedral or Trigonal and Triclinic.

Bravais Lattices

There are only 14 possible crystal lattices, which are called Bravais Lattices.

Cubic Lattice There are three types of lattice possible for cubic lattice.

Primitive or Simple, Body centred, Face centred lattices. In these types of lattices all sides are of equal length. The angles between their faces are 900 in a cubic lattice.

primitive cell

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Answer 2

Types of unit cells in crystal:

1) Primitive Cubic Unit Cell

2) Body-Centered Cubic Unit Cell

3) Face-Centered Cubic Unit Cell

1.) In a Primitive unit cell, atoms are present only at the corners. Every atom at the corner is shared among eight adjacent unit cells. Four unit cells are present in the same layer. Four unit cells in the upper/lower layer. Therefore, a particular unit cell has only 1/8th of an atom. Each small sphere in the following figure represents the centre of a particle which occupies that particular position and not its size. The total number of atoms in one unit cell is one.

2.) In a Body-Centred Cubic Unit Cell, every corner has atoms. There is one atom present at the centre of the. structure It is an open structure. As per this structure atom at the body centre completely belongs to the unit cell within which it is present. The total number of atoms present in one unit cell is two.

3.) In a Face-Centered Cubic Unit Cell, atoms are present in all the corners of the crystal lattice. Also, there is an atom present at the centre of every face, ie. the face centre of the cube. This face-centre atom is shared between two adjoining unit cells. Only 12 of each atom has a place in a unit cell. The total number of atoms present in a unit cell is four.

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