2. Crystal Structure
A solid is defined as the form of matter which exhibits rigidity, a
definite shape and a definite volume.
3. Solids can be classified as crystalline solids and amorphous solids.
Crystalline solids have a long range arrangement of constituent
particles. They have a sharp melting point and are anisotropic in
nature. For example; Quartz , all solid elements (metal and non-
metal).
In amorphous solids, there is a short range order in the arrangement
of particles. They have irregular shape and are isotropic in nature.
They are called as pseudo solids or supercooled liquids. For
example; glass, silica, plastic, polymers.
4. Crystal Lattice Structure
In crystalline solids, the constituent particles are arranged in a
regular pattern throughout the crystal lattice. This regular and
repeating arrangement of points or particles in space is called as
space lattice or crystal lattice structure.
Since crystal lattice is a regular and repeating arrangement of
partials, a small part of the lattice will be sufficient to explain all the
properties and complete crystal lattice.
This smallest part of the crystal lattice, which when repeated in
different directions,
produces a complete crystal lattice, is known as unit cell.
5.
6. Properties of Crystal lattice
In the crystal lattice, each point represents constituent particles (ion or atom or
molecule) and is called as lattice point. These points joined by line to form a whole
crystal lattice. The arrangement of lattice points in a crystal lattice gives the geometry
to a crystal lattice. Crystal lattice can be of two types,
1.Two dimensional lattices
2.Three dimensional lattices.
7. Two dimensional lattices
It is a two dimensional regular arrangement of particles in two
dimensions or on the plane of a paper.
A unit cell with a certain number of particles in it's corner is known as
a primitive unit cell, while a unit cell with corner as well as interior
particles is known as interior unit cell.
8. The type of crystal lattice depends on the type of unit cell.
The complete crystal lattice is produced by repeatedly
moving unit cells in the direction of its edge.
9. 2. Three dimensional lattices
In these type of crystal lattices, the constituent particles are
arranged in a three dimensional space.
10.
11. Unit Cell
Each smallest unit of the complete space lattice or crystal lattice,
which is repeated in different direction to form a complete crystal
lattice structure is called a unit cell. It is just like a thick wall made up
of regularly arranged bricks. Here the thick wall is the crystal lattice
and each brick is a unit cell.
In other words, unit cell is the building block of crystal lattice or
space lattice. A unit cell can be explained by using certain
parameters. These parameters are as follows. The edge of the unit
cell represented by a, b and c. It is dimensions along the three
edges.
12. The angle between the edges are represented by α, β and γ. The
angle between edge b and c is α , the angle between edge a and
c is β, while γ is the angle between edges a and b. Thus there are a
total of six parameters; a , b , c and α, β and γ
13.
14. Types of Unit cells
1.Based upon th parameters of unit cell
The unit cell can be classified into seven different types on THE BASIS
OF the different parameters a, b and c edges and α ,β and ϒ
angles. These seven unit cells are also known as Bravais Unit Cells.
15. 2. Based upon the position of
particles in unit cell
Each Bravais unit cell is further classified into two types on THE BASIS
OF the position of the particles at the corner and center. The unit
cells which have lattice points only at the corner are termed as
primitive unit cells. While the unit cells in which the lattice points are
located at the corner as well as at other positions also are called as
non-primitive or centered unit cell.
16. The non-primitive units cells are further divided into three types on
the basis of lattice points at other sites.
Face centered unit cell: When particles are located at the corner as
well as at the center of each face, it is termed as face centered unit
cell.
End-centered unit cell: In such type of unit cells , particles located at
the corner and at the center of any two opposite faces.
Body centered unit cell: When lattice points are located at the
corner and one particle at the center of unit cell , it termed as body
centered unit cell.
17. Cubic Closest Packed Structure
In crystal lattices all the lattice points or particles are taken as a
sphere. All the spheres are arranged in such a way that they
occupy the maximum available space and leave minimum empty
space between them. The packing of spheres can be of different
types.
18. Close packing in one dimensional
In this packing, particles can arranged only in one dimension. They
are arranged in such a way that they touch each other in a row.
The coordination number is two in this arrangement.
19. Close packing in two dimensional
The two dimensional arrangement of particles in also known as
crystal plane. In this arrangement, the packing of particles can be
done in two different ways.
The spheres of the second row are arranged in such a way that they
are touching the spheres of the first row and are present exactly
below them. Such type of arrangement is termed as 'AAAA' type ,
because all the layers are same. Each sphere touches four other
sphere, hence the coordination number is four. This type of
arrangement is also known as square close packing in two
dimensions.
20. Another type of two dimensional arrangement is known as
hexagonal close packing in two dimensions. In this arrangement,
the second layer of spheres is arranged in the depressions of first
layer. Hence it also represents as 'ABABAB'.
21. Close packing in three dimensional
1. Simple primitive cubic unit cell and simple cubic lattice
This type of packing is made by three dimension packing from
square close packed layers. In square packed layers, all further
layers will be built up such that they are
horizontally as well as vertically aligned with each other. This
arrangement is also written as 'AAAA type'•. There is a simple
primitive cubic unit cell and simple cubic lattice.
