4. INTRODUCTION:
Solids are characterized
by incompressibility,
rigidity and mechanical
strength.
Solids are usually
classified as either
1.Crystalline solids
2.Amorphous solids
5. 1.CRYSTALLINE SOLIDS:
Crystalline solids have definite and ordered arrangement of
the constituents extended over a long distance and is
called a long-range order. They possess a sharp melting
point.
:2.AMORPHOUS SOLIDS
Amorphous solids like glass, rubber etc., although
possessing many characteristics
of crystalline solids such as definite shape, rigidity and
hardness, but are devoid
of a regular internal structure and melt gradually over a
range of temperature. For
this reason they are not considered as true solids but
rather highly supercooled
liquids
6.
7. CRYSTALLOGRAPHY:
A study of internal structure of
crystals. 1913 – English physicists,
Father and Son, William and
Lawrence Bragg developed X-ray
crystallography further by
establishing laws that govern the
orderly arrangement of atoms in
crystal interference and diffraction
patterns. They also demonstrated
the wave nature of X-rays. In 1915
William Bragg and Lawrence Bragg
were awarded Nobel prize for
physics for X-ray analysis of crystal
structure.
8. UNIT CELL:
Unit cell is the smallest fundamental
repeating portion of a crystal lattice
from which the crystal is built by
repetition in three dimension.
Types of Cubic System
Simple cubic
Body centred cubic
Face centred cubic
9.
10. X-RAYS AND CRYSTAL STRUCTURE
X-rays are electromagnetic waves
of very short wavelength. The
wave nature
of X-rays is not confirmed by
diffraction experiment, because a
grating of about
40 million ruling per cm is
required for diffraction
experiment. The preparation of
such a grating is highly impossible.
11. BRAGG’S EQUATION
W.L.Bragg and W.H.Bragg derived a
mathematical relation to determine
interatomic distances from X-ray diffraction
patterns. The scattering of X-rays by crystals
could be considered as reflection from
successive planes of atoms in the crystals.
However, unlike reflection of ordinary light,
the reflection of X-rays
can take place only at certain angles which are
determined by the wavelength of
the X-rays and the distance between the
planes in the crystal. The fundamental
equation which gives a simple relation
between the wavelength of the X-rays,
the interplanar distance in the crystal and the
angle of reflection
12. BRAGG’S EQUATION
Bragg’s equation is nλ = 2d sinθ
where n is the order of
reflection
λ is the wavelength of X-rays
d is the interplanar distance in
the crystal
θ is the angle of reflection
13. SIGNIFICANCE OF BRAGG’S
EQUATION
If we use X-rays of known
wavelength (λ), then the interatomic
distance (d) in an unknown crystal
can be calculated, crystal whose
interatomic distance ‘d’ is known,
then the wavelength of X-rays can be
calculated.
14. BRAGG’S SPECTROMETER METHOD
This method is one of the important
method for studying crystals using Xrays. The apparatus consists of a X-ray
tube from which a narrow beam of Xrays is allowed to fall on the crystal
mounted on a rotating table.The
rotating table is provided with scale
and vernier, from which the angle of
incidence, θ can be measured.
15.
16. TYPES OF CRYSTALS
Crystals are classified into the
following four types depending upon
the
nature of the units which occupy the
lattice points.
1. Molecular Crystals 2. Covalent
Crystals
3. Metallic Crystals 4. Ionic Crystals
17. Molecular Crystals
The lattice points in molecular
crystals consist of molecules
which do not carry any charge.
The forces binding the
molecules together are of two
types
a.Dipole-dipole interaction
b.Vanderwaal’s forces
18. Dipole-dipole forces
occur in solids which
consists of polar
molecules e.g., ice.
The Vanderwaal’s
forces are more
general and occur in
all kinds of
molecular solids.
19.
20. Covalent Crystals
The lattice in covalent crystals
consists of atoms linked
together by acontinuous
system of covalent bonds.
Diamond is a good example
for this type
21.
22. Metallic Crystals
Metallic crystal consists of an
assemblage of positive ions
immersed in a sea of mobile
electrons. The force that binds a
metal ion to a number of electrons
within its sphere of influence is
known as metallic bond. This force of
attraction is strong and is thus
responsible for a compact solid
structure of metals.
23.
24. Ionic Crystals
In ionic crystals, the
units occupying lattice
points are positive
and negative ions.
Each ion of a given
sign is held by
coulombic forces of
attraction to all ions
of opposite sign. The
forces are very strong.
