Solid State 3

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SOLID STATE 3
 Point defects- these are irregularities or deviations from ideal
arrangement around a point or an atom in a crystalline substance.
 Line defects- these are the irregularities from ideal arrangement in
entire rows of lattice points.
 Point defects are of 3 types (i) stoichiometric defects also called as
intrinsic or thermodynamic defects. (ii) impurity defects, (iii) nonstoichiometric defects.
 Vacancy defects- they occurs when position that should contains atoms
or ions are vacant. This results in decrease in density. Schottky defect
is basically a vacancy defect shown by ionic solids.
 Interstitial defect- it occurs when some constituent particles occupy an
interstitial site. This defect increases the density of the substance.
Frenkel defect is just like the interstitial defect shown by the ionic
solids.
 Frenkel defect (dislocation defect) occurs when atoms or ions at the
lattice points displace to an interstitial site creating a vacancy. It does
not change the density of the substance. It is shown by the ionic
substances in which there is the large differences in the size of ions,
e.g. ZnS, AgCl, AgBr, and AgI due to small size of Zn +2 and Ag+ ions.
 Schottky defect- occurs due to missing of atoms or ions from the lattice
point. In this defect the density of the substance decreases. It is shown
by NaCl, KCl, CsCl and AgBr.
 AgBr show both, frenkel as well as schottky defects.
 F-centres are the ionic site occupied by the un pair electrons. They
impart colour to the alkali metal halide crystals. Excess of Na makes
NaCl crystal yellow, excess of lithium makes LiCl crystal pink, excess of
K makes KCl crystal violet.
 Impurity defects are shown by the molten NaCl having little amount of
SrCl2, also by solid solution of CdCl2, and AgCl.
 Metal excess defect due to anionic vacancies is shown by alkali halides
like NaCl and KCl.
 Metal excess defect due to presence of extra cations at interstitials
sites is shown by zinc oxide when it is heated.
 On heating, zinc oxide (white) loses its oxygen and turns yellow.
 Metal deficiency defects is shown by FeO.
 In conductors e.g. metals, valance band is partially filled or it overlaps
with conduction band, this results in easy flow of electrons. Therefore
metals shows conductivity. In insulators, there is a large energy gap
between filled valance band and empty conduction band (forbidden



















zone), due to this electron cannot jump to it, therefore, these
substances show very small conductivity.
In semiconductors, there is a small energy gap between valance band
and conduction band. Therefore, some electrons may jump to
conduction band and shows some conductivity. Silicon and Germanium
shows this type of behavior and are called intrinsic semiconductors. An
intrinsic semiconductor material is chemically very poor possesses
poor conductivity.
Conductivity of intrinsic semiconductors is increased by adding an
appropriate amount of suitable impurity (doping) and intrinsic
semiconductors are obtained, which are of two types n-type and p-type
semiconductors. If Si or Ge is doped with electron rich impurities (gp.
15 elements like P,As) , n-type semiconductors is obtained. If Si or Ge is
dopped with electron deficient impurities ( gp.13 elements like B,Al or
Ga), p-type semiconductor is obtained.
On the basis of magnetic properties, substances can be classified as,
paramagnetic, diamagnetic, ferromagnetic, antiferromagnetic, and
ferromagnetic.
Diamagnetic substances are feebly repelled by magnetic fields. These
substances have all paired electrons, e.g. TiO 2, NaCl.
Paramagnetic substances are attracted by magnetic field. These
substances has at least 1 unpaired electron, e.g. Cu +2, TiO.
Ferromagnetic substances are attracted strongly by magnetic field, e.g.
Fe, Co, Ni.
In solid state metal ions of ferromagnetic substances are grouped
together in small regions called domains.
Ferromagnetic substances have permanent magnetism. When
ferromagnetic substance is placed in magnetic field, dipoles are
aligned in the same direction ↑ ↑ ↑ ↑, e.g. Fe, Ni, Co.
Anti ferromagnetic substances have domain structure similar to
ferromagnetic substances but their domains are oppositely oriented,
therefore, these substances have net dipole alignment zero, e.g., MnO,
MnO2 (↑↓↓↑)
Ferromagnetic substances are weakly attracted by magnetic field as
compared to ferromagnetic substances. Their domains are aligned in
parallel and anti-parallel directions in unequal numbers, therefore,
these substances have net dipole, e.g. Fe3O4, MgFe2O4, ZnFe2O4(↓↑↑↑↓↓↑)
Piezoelectric crystals produce electricity due to mechanical stress.

POINT DEFECT
→STOICHIOMETRIC DEFECT
 Vacancy defect

 Interstitial defect
→IMPURITY DEFECT
→NON-STOICHIOMETRIC DEFECT
 Metal excess defect
 Due to anionic vacancies
 Due to extra cations
 Metal deficiency defect

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