Crystals are solids with a repeating structure called a unit cell. A pure crystal has identical, uniformly aligned molecules throughout. Common examples of crystals include salt, sugar, and snowflakes. Crystals contain defects such as vacancies, interstitials, and improper stoichiometry. Stoichiometric defects like Schottky and Frenkel involve missing or misplaced ions while maintaining the proper cation to anion ratio. Non-stoichiometric defects involve an improper ratio, caused by excess metal or nonmetal. These defects impact properties like conductivity, color, and stability.

1. by
Dr.S. Manimekalai
Assistant professor of chemistry
E.M.G.Yadava women’s college
Madurai

2. What is Crystal?
A pure crystal is the substance in which all the
molecules are perfectly identical and the alignment of
molecules with each other is perfectly uniform
throughout the substance."
Crystals are solids with a structure (called a unit cell)
that is repeated along all the axes of the
crystal. Departures from "purity" on a crystal can be in
composition or in structure
the word "crystal" comes from the Ancient Greek
word krustallos, which means both "rock crystal" and
"ice." The scientific study of crystals is
called crystallography.

3. Examples of Crystals
crystals are table salt (sodium chloride or halite crystals), sugar (sucrose),
and snowflakes. Many gemstones are crystals, including quartz and
diamond.
Compositional impurities occur when some of the atoms that are supposed
to be in the crystal (e.g., carbon atoms in a diamond) are replaced by atoms
of another element (e.g., nitrogen in a diamond causes a yellow or brownish
color).
some unit cells have a different composition than other unit cells.

4. Diamond sugar crystals

6. Imperfections in solids
 A solid consists of an aggregate of large number of small crystals.
 These small crystals have defects
 crystallisation process occurs at fast or moderate rate
 Single crystals are formed when the process of crystallisation occurs at
extremely slow rate
 These crystals are not free of defects
 Defects are irregularities in the arrangement of constituent particles
An ideally perfect crystal is one which has the same unit cell and contains
the same lattice points throughout the crystal. The term imperfection or
defect

9. Stoichiometry Defects
The compounds in which the number of cation and
anions are exactly in the same ratio as represented by
their chemical formula are called stoichiometric
compounds. The defects that do not disturb the ratio of
cations and anions are called stoichiometric defect.
These are of four types
Vacancy Defect
Interstitial Defect
Schottky Defect
Frenkel Defect

10. Vacancy Defect
•When some of the lattice sites are vacant, the crystal is said to have
vacancy defect .
•This results in decrease in density of the substance.
•This defect can also develop when a substance is heated.
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.
the substance

11. Schottky Defect
 If in an ionic crystal of the type A+ B-, equal number of cations and
anions are missing from their lattice. It is called Schottky defect.
 This type of defect is shown by highly ionic compounds which have
high Co – ordination number and Small difference in the sizes of
cations and anions
 This defects are require less energy for their formation
 A few examples of ionic compounds exhibiting Schottky defect are
NaCl, KCl, KBr and CsCl.

12. Schottky Defect
hole produced by Na+ (cation) missing
hole produced by Cl-
( anion) missing

13. Consequences of Schottky Defect
•As the number of ions decreases as a result of this defect, the
mass decreases whereas the volume remains the same. Hence
density of the solid decreases
•The crystal begins to conduct electricity to a small extent by
ionic mechanism
•The presence of too many voids lowers the lattice energy and
stability of the crystal
•The presence of holes permits easy movement of ions in the
crystal which change their position with one another. This
phenomenon of diffusion in solids

14. Frenkel Defect
If an ion is missing 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.
This type of defect is called Frenkel defect. Since cations
are usually smaller it is more common to find the cations
occupying interstitial sites.
This type of defect is present in ionic compounds which
have low co ordinations number
Larger difference in size of cation and anions
Compounds having highly polarising cation and easily
polarisable anion. A few examples of ionic compounds
exhibiting this defect are AgCl, AgBr, AgI, ZnS etc.
As the temp increased, the no.of frenkel defects also ↑sed

15. Frenkel Defect

16. Consequences of Frenkel defect
•As no ions are missing from the crystal lattice as a whole, therefore
density of the solid remains the same
•The closeness of like charges tends to increases the dielectric
constant of the crystal
•The crystal conducts electricity to a small extent by ionic mechanism
•The presence of holes lowers the lattice energy and stability of the
crystal
•The presence of holes permits easy movement of ions in the crystal
which change their position with one another. This phenomenon of
diffusion in solids

17. Non – Stoichiometric Defects
If as a result of imperfection, the ratio of number of cation to
anion becomes different from that indicated by the ideal chemical
formula, the defects are called non – stoichiometric defects.
 These defects arise either due to excess of metal atoms or non
metal atom or presence of impurities / foreign particle.

18. Metal (positive ions) excess Defects
It is produced due to the presence of excess of positive ions in the
crystal lattice
• one of the negative ions is missing from its real position, leaving a
hole and this hole is occupies by an extra free electron
•The negative ions leaves the surface in the form a gas
•Example: 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-stoichiometric
compound of sodium chloride in which there is a slight excess of
sodium ions.
•sodium metal gets doped into sodium chloride crystal which, due to
the crystal energy, gets ionised into Na+ and e–. This electron
occupies a site that would otherwise be filled by a chloride ion.
.

19. As a result, the non – stoichiometric form of sodium
chloride appears coloured. Because of this, the sites occupied by the
extra electrons are known as colour centres. These are also called F-
centres. This name comes from the German
word Farbe meaning colour.
The non-schiometric sodium chloride may be represented by the
formula Na(1+d)Cl where d is the excess sodium metal doped in the
crystal because of its exposure to sodium vapour.
It is similar to that of schottky defects. But this contains one hole
while the schottky has two holes
Another common example of metal excess defects is the formation
of a magenta coloured non-stoichiometric compound of potassium
chloride by exposing the crystals of KCl to K metal vapour.

21. Consequences of Metal Excess Defects
This defect conduct electricity with the help of free
electrons.
Number of free electrons is small, it conduct electricity
also small
The amount of electricity conducted by such crystal
smaller than that conducted by metals, fused salts and
water solution of salts
The crystals having this type of defects are called
semiconductors
the crystals are coloured (free electrons)
The free electrons absorb white visible light to higher
energy levels , the compounds appear coloured.
Ex: zinc oxide is white in cold appears yellow when hot

22. Metal Deficiency Defects
It is produced by the presence of excess of negative ions in the
crystal lattice
• 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, ie A2+.
• A deficiency of the metal ions although the crystal as a whole is
neutral.
•This type of defect is generally found amongst the compounds of
transition metals which can exhibit variable valency.
•Crystals of FeO, FeS and NiO show this type of defects.

23. Metal deficiency defect due to
the missing of a cation cation carrying one
extra positive charge

24. Consequences of Metal deficiency Defects
One of the cation A+ is changed into A2+ cation
through the movement of an electron.
This is called movement of positive hole
This defects behaves as semiconduct

25. SrCl2 added to molden NaCl. Hence some Na+ ions substituted
by Sr2+. It increases electrical conductivity.

26. Conclusion
all types of point defects result in the creation of
vacancies or ‘holes’ in the lattice of the crystals.
The presence of holes lowers the density as well as the
lattice energy or the stability of the crystals.
 The presence of too many holes may cause a partial
collapse of the lattice.