Point defect in solids

point defects are the irregularities from ideal arrangement around a point in a crystalline substance.

1. Defect in solids
Line defect & Point defect

2. Point & Line defect

3. Point defects in solids

4. Stoichiometric defects
These are the point defect that do not disturb the
stoichiometry of the constituent particles in the
crystal.
They are called intrinsicor thermodynamic
defect.
This defect is of two types.

5. Types of Stoichiometric defect
Non-ionic compound
(a) Vacancy defect
(b) Interstitial defect
Ionic compound
These solids must always
maintain electrical neutrality.
(a) Frenkel defect
(b) Schottky defect

6. Vacancy Defect
When some of the lattice
sites are vacant,
thecrystal is said to have
vacancy defect .
This results indecrease
in density of the
substance.
This defect can also
developwhen
asubstance is heated.

7. Interstitial defect
When some constituent
particles(atoms or
molecules) occupy an
interstitial site,the
crystal is said to have
interstitial defect
This defect increases the
density of thesubstance.


8. Frenkel defect
Smaller size ion is
missing (dislocating)
from its correct lattice
sites (causing a vacancy
or a hole) and occupies
an interstitial site.
Electrical neutrality as
well as stoichiometry of
the compounds are
maintained.
Hole

9. Frenkel defect
Dislocation defect
No change in density of
solid
Large difference in the
size of cationand anion
Example : ZnS, AgCl,
AgBr,

10. Schottky defect
In the ionic crystal of the
type A
+
B
-
, equal number
of cationsand anions are
missing from their lattice
point . It is called
Schottky defect.
It maintain the electrical
neutrality of the
compound.

11. Schottky defect
Density of the crystal
decreases.
The size of the Cation
and anion is almost
similar.
Example : NaCl, KCl,
CsCl, AgBr.
Example: In NaCl,
there are approximately
10
6
Schottky pair per cm
3
at room temperature. In
1 cm
3
there are about
10
22
ions. Thus, there is
one Schottky defect per
10
16
ions.

12. Difference

13. Impurity Defect
If molten NaCl is
crystallized with a little
amount of SrCl
2, some of
the sites of Na
+
ions are
occupied by Sr
2+
ion.
Each Sr
2+
ion replaces
two Na
+
ion.
Number of vacancy
produced, is equal to that
of Sr
2+
ions.

14. Non-Stoichiometric defect
Metal Excess defect
It is of two types:
(a)Metal excess defect due to
anionic vacancies
(b)Metal excess defect due to
the presence of extra
cation
Metal deficiency
defect

15. Non-Stoichiometric defect

16. Metal excess defect due to anionic vacancies
A crystal of NaCl is heated in
sodium vapour, it acquires a
yellow colour.
This yellow colour is due to the
formation of a non-
stoichiometriccompound of
sodium chloride in which there
is a slight excess of sodium
ions.
The anionic sites occupied by
the unpaired electron are called
F -centres
F -Centre

17. Metal excess defect due to anionic vacancy

18. The colour results by the
excitation of these
electrons, when they
absorb energy from visible
light.
LiClcrystal appear pink.
KClcrystal appear violet.

19. Metal excess defect due to the presence of extra cation
On heating ZnOloses O
2
and turns yellow.
Formula Zn
1+xO
Zn
2+
and electron moves
to the different
interstitial sites.

21. Metal Deficiency defect
One of the positive ions is missing from its lattice
site and the extra negative charge is balanced by
some nearby metal ion acquiring two charges
instead of one.
This type of defect is generally found in the
compounds of transition metals which can
exhibit variable valency.
Crystals of FeO, FeSand NiOshow this type of
defects.

23. Composition is Fe
0.95O

24. THANKS