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1. JFC Crystal Defects (Solid State Chemistry) By Rawat Sir [M.Sc. Chemistry, 3 times NET (JRF), GATE ]
rwtdgreat@gmail.com
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Atomic Imperfections / Point Defects:
 If the defect arises due to the missing, displaced or extra atoms, the imperfection is named as a point defect.
 These defects are the result of imperfect packing during the original crystallisation or they may arise from
thermal vibrations of atoms at higher temperatures because with increase in thermal energy, entropy increases.
Type of point defects – A. Stoichiometric defects, B. Non – stoichiometry defects and C. Impurity defects
A. 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. “
OR
“The defects that do not disturb the ratio of cations and anions are called stoichiometric defect.”
1. 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.
2. Interstitial Defect:
 Interstitial defect is shown by non-ionic solids
 This type of defect is created when some constituent particles (atoms or molecules)
occupy an interstitial site of the crystal.
 This defect increases the density of the substance.
3. Schottky Defect:
 Schottky defect is basically a vacancy defect shown by ionic solids.
 In this defect, an equal number of cations and anions are missing to maintain electrical
neutrality.
 It is shown by the crystals having high Co – ordination number.
 Ionic substances containing similar sized cations and anions show this type of defect.
 For example: NaCl, KCl, CsCl, AgBr etc.
Consequences of Schottky defect
 It decreases the density of a substance.
 The crystal begins to conduct electricity to a small extent by ionic mechanism
 The presence of too many voids decreases lattice energy and the stability of the crystal.
1. Vacancy
defect
2. Interstitial
defect
3. Schottky
defect
4. Frenkel
defect
Defects in Solids
Impurity
defects
Non-
Stoichiometric
defects
Stoichiometric
defects
1. Metal
Excess defect
2. Metal
Deficiency
defect© Rawat’s JFC (Just 4 Chemistry)

2. JFC Crystal Defects (Solid State Chemistry) By Rawat Sir [M.Sc. Chemistry, 3 times NET (JRF), GATE ]
rwtdgreat@gmail.com
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4. Frenkel defect:
 Ionic solids containing large differences in the sizes of ions show this type of defect.
 When the smaller ion (usually cation) is dislocated from its normal site to an interstitial site,
Frenkel defect is created.
 It is observed in the ionic crystals which have low co-ordinations number
 It creates a vacancy defect as well as an interstitial defect.
 It is present in ionic compounds having highly polarising cation and easily polarisable anion
 Frenkel defect is also known as dislocation defect.
 Ionic solids such as AgCl, AgBr, AgI, and ZnS show this type of defect.
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
B. 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.
1. Metal Excess Defects
(i) Metal excess defect occurs due to Anionic vacancies, THE COLOUR CENTRES (F-center; Ger: farbenzentre):
[Anionic vacancy which is occupied by an electron]
When the anionic vacancies produced in a crystal are occupied by unpaired electrons, the ionic sites are called F-centres.
These unpaired electrons impart color to the crystals, because they can show transition from its ground state (HOMO) to
an excited state (LUMO) in the visible range.
For example,
 When crystals of NaCl are heated in an atmosphere of sodium vapours, the
sodium atoms are deposited on the surface of the crystal.
 The Cl
-
ions diffuse from the crystal to its surface and combine with Na atoms,
forming NaCl.
 During this process, the Na atoms on the surface of the crystal lose electrons.
Na (g)  Na+
(g) + e-
 These released electrons diffuse into the crystal and occupy the vacant anionic
sites, creating F-centre and the crystal of NaCl acquires yellow color.
 This yellow color is due to the formation of a non-stoichiometric compound of sodium chloride in which there is a
slight excess of sodium ions, although the crystal as a whole is neutral, hence F- Centre is a type of metal
excess defect.
 A little reflection would show that there are six Na+
sites adjacent to the vacant site occupied by the electron.
 The extra electron is thus shared between all the six Na+
ions which imply that this electron is not localised at the
vacant Cl–
site, but this electron is similar to the delocalised - electrons present in molecules containing
conjugate double bonds.
 Light is absorbed when this delocalised electron makes an easy transition. As a result, the non – stoichiometric
form of sodium chloride appears colored.
 Because of this the sites occupied by the extra electrons are known as color centres. These are also called F-
centres. This name comes from the German word Farbe meaning color.

3. JFC Crystal Defects (Solid State Chemistry) By Rawat Sir [M.Sc. Chemistry, 3 times NET (JRF), GATE ]
rwtdgreat@gmail.com
www.slideshare.net/RawatDAgreatt
Google+/blogger/Facebook/Twitter-@RawatDAgreatt +919808050301, +917017378183
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 The non-stoichiometric 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.
 Other examples- LiCl acquires pink color, Magenta/Violet color of KCl
 Solids containing F- centres are Photo- Conducting.
 Since the conduction is due to electron hence act as n-type semiconductors.
 Greater the no. of F- centres greater will be the intensity of color.
 The solids containing F- centres are paramagnetic in nature.
(ii) Metal excess defect due to extra cation-
 In this type of defect some extra cation (from the outside of the crystal) occupies
some interstitial positions and the electron occupies another interstitial site to
maintain electrical neutrality.
 The defect may be visualised as the loss of non-metal atoms which leave their electrons behind.
 Density of the crystal increases.
 This defect is similar to Frenkel defect, but the difference is that the density increases.
 The common example is ZnO which is white in color at room temperature but on heating it loses oxygen and
turns yellow, because of electron transition.
ZnO  Zn2+
+ O2 + 2e-
2. Metal Deficiency Defect:
(i) Metal Deficiency Defect due to Cationic Vacancies:
 This type of defect is generally found in the compounds of transition metals
which show variable oxidation states.
 In this type of defect some cations are missing from their lattice position and
hence cationic vacancies are created.
 The extra negative charge is balanced by some other cations of the lattice
which are converted into their higher oxidation state.
 Thus, there is a deficiency of the metal ions although the crystal as a whole is
neutral.
 This defect is like Schottky defect as the density of the crystal decreases.
 The common examples of compounds having this defect are ferrous oxide (FeO), ferrous Sulphide (FeS), nickel
oxide (NiO) etc.
(ii) Metal Deficiency Defect due to Extra Anions:
 In this type of defect some extra anion (from the outside of the crystal) occupies
some interstitial site.
 The extra negative charge is balanced by some other cations of the crystal which
are converted into their higher oxidation state.
 This defect is similar to Frenkel defect but the difference is that the density
increases.
 Such type of defect is not common because the negative ions usually very large
and they cannot easily fit into the interstitial sites.
 Eg. VO2

4. JFC Crystal Defects (Solid State Chemistry) By Rawat Sir [M.Sc. Chemistry, 3 times NET (JRF), GATE ]
rwtdgreat@gmail.com
www.slideshare.net/RawatDAgreatt
Google+/blogger/Facebook/Twitter-@RawatDAgreatt +919808050301, +917017378183
4
C. Impurity Defects:
 This type of defect arises when some foreign particles are mixed with the original crystal.
 Foreign particles, those are mixed, known as impurity.
 This process of introducing defects by adding small amount of foreign impurity in the host crystal is called doping.
 Foreign particles occupy the position of host particles in the crystal lattice or they occupy the vacant interstitial sites.
 Addition of impurities, change the properties of the crystal.
 When the impurity is present at the lattice point, substitutional solid solution is obtained. Its formation depends upon
the electronic structure of the impurity.
 When the impurity occupies an interstitial site, an interstitial solid solution is obtained. Its formation depends upon
the size of the impurity.
(a) Impurity defect in ionic solids
 It is introduced by adding impurity of ions in ionic solids.
 If the impurity ions are in different oxidation state from that of host ions,
vacancies are created.
For example-
 When SrCl2 is added as impurity in the crystal of NaCl, Sr2+
ion occupies some of
the lattice points and displaces Na+
ions.
 Every Sr2+
ion results in the removal of two Na+
ions to maintain the electrical
neutrality and as a result, one of these lattice points is occupied by Sr2+
ion and
at other cationic vacancy is generated.
 Cationic vacancy generated = Sr2+
doping
 These vacancies result in higher electrical conductivity of the solid.
Another example-
Addition of CdCl2 as impurity to AgCl. Here, Cd2+
ion occupies some of the lattice points of Ag+
ions.
(b) Impurity defect in covalent solids-
Examples- n type and p type semiconductors.
Doping increases the electrical conductivity of the material. Thus, impurity-doped Si and Ge act as semiconductors.