4. A crystal isan organic,inorganic or biological substance
with a specific chemical composition and definite 3D
intermolecular pattern.
Ideal
structure
of solid
Deviations from
ideal structure
Defects
Every lattice point
has the same
environment
7. • If the deviation occurs because of missing atoms, displaced atoms
or extra atoms, the imperfection is named as point defect.
Stoichiometric
defects
Non-stoichiometric
defects
Vacancy
Interstitial
Frenkel
Schottky
Metal excess
Metal
deficiency
Point defect
8. STOICHIOMETRIC DEFECTS
Number of cations and anions are exactly in the same
ratio as represented by their chemical formula called
stoichiometric compounds.
The defects that do not disturb the ratio of cations and
anions are called stoichiometric defects.
9. Atoms missing from an atomic site.
Occur due to imperfect packing during crystallisation.
This results in decrease in density of the substance.
Number of vacancy defects depends on temperature.
10. Addition of an extra atom within a crystal structure.
This defect increases the density of the substance.
Causes atomic distortion.
Vacancy and interstitials are inverse phenomenon.
11. Cation (being smaller) get displaced to interstitial voids.
Combination of vacancy and interstitial atom.
No change in the density.
E.g. AgI, CaF2.
12. Pair of anion and cation vacancies.
Inorder to maintain electrical neutrality, the numberofmissing
cations and anions are equal.
It also decreasesthe density ofthe crystal.
E.g. Alkali halides suchas NaCl,KF etc.
14. The equilibrium number of vacancies for a given quantity of
material depends on and increases with temperature as follows:
= 𝐞𝐱 𝐩 −
𝑸 𝑽
𝒌𝑻
𝑵 𝑽
𝑵
Nv/N
T
Defect concentration
exponential
dependence
15. • The ratios of number of cation to anion becomes different
from that indicated by the ideal chemical formula, the
defects are called non-stoichiometric defects.
• Arises due to excess of metal atoms or non-metal atom or
presence of impurities or foreign particles.
16. Anionicvacancy:
Anion misses out from its
usual position
The empty spaces are
occupied by electrons.
The space that electron
invades.
Eg: NaCl exhibits yellow color
on being heated.
Extra cations:
• On being heated many
compounds gives extra
cations.
• The cations occupies the
interstitial sites.
• Eg: When ZnO is heated, it
looses oxygen.
In these defects the positive ions are in excess.
These may arise due to the following ways:
17.
18. The presence of defects which affect the color can make these crystals
valuable as Gems as in Ruby (Cr replacing a small fraction of the Al in
Al2O3).
Silicon crystals are doped with Nitrogen group of elements P and As
or boron group of elements B, Al, etc. to form semiconductors.
Screw type dislocations drives the growth of zinc oxide nanotubes
which are used in micro-electronics, solar powers , batteries etc.
Dislocations in crystal are fundamental to the growth an are
characteristics of all crystalline materials
19. No crystal is perfect.
Most of the crystal have defects.
Concentrationor degree of defects is very important.
Defectsin crystals are usefulmost of the time.
