TEXT BASED QUESTIONS AND ANSWERS IN SOLID STATE
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What are imperfections or defects? How they arise?
The defects are basically irregularities in the arrangement of constituent particles. This happens
when crystallisation process occurs at fast or moderate rate.
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What are point defects? What are its various types?
Ans: Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in
a crystalline substance Point defects are of three types : (i) stoichiometric defects (ii) impurity defects
and (iii) non-stoichiometric defects.
Q.
What are stoichiometric defects?
Ans: Stioichiometric defects are the point defects that do not disturb the stoichiometry of the solid. They
are also called intrinsic or thermodynamic defects. Basically these are of two types, vacancy defects
and interstitial defects.
Q.
What are the vacancy defects? How they arise and which property of the solids are affected by
them?
Ans: 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.
Q.
Write short note on interstitial defects.
Ans: 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.
Q.
What are the stoichiometric defects present in metallic solids and which one in ionic solids?
Ans: Vacancy defects and interstitial defects are the stoichiometric defects which arises in metallic solids
while Schottky and Frenkel defects are the stoichiometric defects which arises in ionic solids.
Q.
Write a short note on Frenkel defect.
Ans: Frenkel defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal
site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its
new location. Frenkel defect is also called dislocation defect. It does not change the density of the
solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of
ions, for example, ZnS, AgCl, AgBr and AgI show Frenkel defect due to small size of Zn2+ and Ag+
ions.
Q.
What is Schottky Defect?
Ans: It is basically a vacancy defect in ionic solids. Schottky defect is shown by ionic substances in which
the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. Schottky
defect decreases the density of the substance.
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Name a substance in which both Schottky and Frnekel defect arises.
Ans: AgBr shows both, Frenkel as well as Schottky defects.
Q.
How the addition of SrCl2 can lead to impurity defect in NaCl crystal?
Ans: When molten NaCl containing a little amount of SrCl2 is crystallised, some of the sites of Na+ ions
are occupied by Sr2+. One Sr2+ can introduce one cation vacancy. Similarly the addition of CdCl2 into
AgCl introduces the impurity defect in AgCl
Q.
What are Non-Stoichiometric Defects?
Ans: The defects which alters the stoichiometry of the ionic solids are called as non-stoichiometric
defects. These defects are of two types: (i) metal excess defect and (ii) metal deficiency defect.
Q.
How Metal excess defect due to anionic vacancies can be created in alkali metal halides?
Which properties of the solids are affected by these defects? Give examples.
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When crystals of alkali metal halide like NaCl are heated in an atmosphere of sodium vapour, the
sodium atoms are deposited on the surface of the crystal. The Cl– ions diffuse to the surface of the
crystal and combine with Na atoms to give NaCl. The released electrons due to the ionization of
Na(g) atoms diffuse into the crystal and occupy anionic sites vacated by Cl— ions.. As a result the
crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called Fcentres. They impart yellow colour to the crystals of NaCl. Similarly, excess of lithium makes LiCl
crystals pink and excess of potassium makes KCl crystals violet (or lilac).
How Metal excess defect due to interstitial cations can be created?
Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow.
ZnO
 Zn2+ + ½ O2 + 2e
Now there is excess of zinc in the crystal and its formula becomes Zn1+xO. The excess Zn2+ ions
move to interstitial sites and the electrons to neighbouring interstitial sites. Non stoichiometric ZnO
is yellow in colour.
Q.
How the metal deficiency defects are created? What are its consequences?
Ans: Metal deficiency defects arise when some cation having higher valency than the cation already
present in the stoichiometric compound is added. For example in stoichiometric FeO, the Fe 2+ and
O2— ions are present in the ratio 1:1. If two Fe3+ ions replace two Fe2+ ions then the two positive
charges becomes excess. The charges can be balanced by cation vacancy created when one Fe 2+
ion misses from its lattice site. The total Fe ions (Fe2+ and Fe3+) thus becomes less and leads to
metal deficiency defect.
Q.
Classify the solids on the basis of their electrical conductivity.
Ans On the basis of electrical conductivity; solids can be classified into three types :
(i) conductors (ii) semi-conductors (iii) insulators.
(i)
Conductors. The solids which allow the passage of electric current are called conductors.
Electrical conductivity of metals is very high and is of the order of 107ohm-1 cm-1.
(ii)
Semi-conductors. The solids whose conductivity lies between those of typical metallic
conductors and insulators are called semiconductors. The semi-conductors have conductivity
in the range of 10-6 to 104 ohm-1 cm-1. The conductivity of semi-conductors is due to the
presence of impurities and defects.
(iii)
Insulators. The solids which do not allow the passage of electric current through them are
called insulators. For example, wood, sulphur, phosphorus, rubber, etc. The insulators have
conductivity in the order of 10-10 ohm-1 cm-1 to10-20 ohm-1 cm-1.
Q What happens to the electrical conductivity of metals and semiconductors with temperature? Ans.
The electrical conductivity of metals decrease with increase in temperature while the electrical
conductivity of semiconductors increase with increase in temperature.
Q.
What are n-type and p-type semiconductors? List their important uses.
Ans. n-type semiconductors: When a silicon crystal is doped with a group-15 elements, such as P, As, Sb
or Bi, it is called an n-type semiconductor.
p-type semiconductors: Group-13 doped crystals of silicon are called a p-type semiconductors since
holes (positive in charge) appear to be responsible for the semiconducting properties.
Various combinations of n-type and p-type semiconductors are used to make electronic components, for
example a diode is a combination of p- and n-type semiconductors and is used as a rectifier.
Q.
Explain the conductance of solids on the basis of band theory.
Ans. Metals conduct electricity in solid as well as molten state. The conductivity of metals depend upon
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the number of valence electrons available per atom. The atomic orbitals of metal atoms in the metal
crystal form molecular orbitals which are so close in energy to each other that they form a band. If
this band is partially filled or it overlaps with a higher energy unoccupied conduction band, then
electrons can flow easily under an applied electric field and the metal shows conductivity.
If the gap between filled valence band and the next higher unoccupied band (conduction band) is
large, electrons cannot jump to it and such a substance has very small conductivity and it behaves
as an insulator.
In case of semiconductors, the gap between the valence band and conduction band is small.
Therefore, some electrons may jump to conduction band and show some conductivity. Electrical
conductivity of semiconductors increases with rise in temperature, since more electrons can jump
to the conduction band. Substances like silicon and germanium show this type of behaviour and are
called intrinsic semiconductors.
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What are group 12-16 and 13-15 compounds?
Germanium and silicon are group-14 elements and have, therefore, a characteristic valence of four
and form four bonds as in diamond. Solid state materials are prepared by the combination of
elements of group-13 and 15 or 12 and 16 to simulate average valence of four as in Ge or Si. Typical
of group 13-15 compounds are InSb, AlP and GaAs. Gallium arsenide(GaAs) semiconductors have
very fast responses. ZnS, CdS, CdSe and HgTe are examples of group 12-16 compounds. In these
compounds, the bonds are not perfectly covalent and the ionicity depends on the
electronegativities of the two elements.
What is a photodiode?
Photo diode is an extrinsic semiconductor which conducts electricity when light falls on it.
List some transition metal oxides which resemble metals.
Some of the transition metal oxides show electrical properties like metals. TiO, CrO2 and
ReO3 behave like metals. Rhenium oxide, ReO3 is like metallic copper in its conductivity and
appearance.
Define ferromagnetic, ferrimagnetic and antiferromagnetic substances.
Ferromagnetism: A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very
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strongly by a magnetic field. Such substances are called ferromagnetic substances. Besides strong
attractions, these substances can be permanently magnetised. The magnetic field lines in such cases
are permanently oriented in one direction superimposing each other and parallel to each other.
Antiferromagnetism: Substances like MnO showing antiferromagnetism have domain structure
similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each
other's magnetic moment.
Ferrimagnetism: Ferrimagnetism is observed when the magnetic moments of the domains in the
substance are aligned in parallel and anti-parallel directions in unequal numbers. They are weakly
attracted by magnetic field as compared to ferromagnetic substances. Fe3O4 (magnetite) and ferrites
like MgFe2O4 and ZnFe2O4 are examples of such substances. These substances also lose
ferrimagnetism on heating and become paramagnetic.
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What are isotrophic and anisotrophic substances?
The substances which have similar properties like electrical and light conductivities in all directions
are called as isotrophic substances. On the other hand the substances which have different
properties in different direction are called as anisotrophic substances. Crystalline solids are
anisotrophic due to the orderly arrangement of particles as shown below.
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What is Bravis lattice?
A regular three dimensional arrangement of points in space is called a crystal lattice. There are only
14 possible three dimensional lattices. These are called Bravais Lattices.
How NaCl type of crystal can be converted into CsCl type and vice versa?
The NaCl type of crystal can be converted into CsCl type by applying pressure and the CsCl type of
crystal can be converted into NaCl by increasing the temperature.
What is effect of temperature and perssure on crystal structure of NaCl and CsCl ?
At ordinary temperature, in NaCl structure, the coordination number Na+ and Cl– ions are 6. If
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pressure is increased, NaCl will take the structure of CsCl i.e., the coordination numbers of Na +
and Cl– ions will become 8 i.e., with the increase in pressure, coordination number is also
increased. CsCl structure will change to NaCl structure at high temperature i.e., 7760 K. Hence
increase in temperature will decrease the coordination number.
Q.
What do you mean by Intrinsic semiconductor ?
Ans. All the four valance electrons of silicon are involved inbond formation. Because of the absence of
free electrons it cannot conduct the electricity and behave as an insulator. But at higher electrons
electric conductance is possible and it is known as intrisic semiconductor. Intrinsic semiconductors
are those insulators which conduct electric current at high temperature like silicon or germanium or
when they silicon or germanium or when they are subjected to electromagnetic radiations.
Q.
How will you distinguish between the following terms. :
(a)
Hexagonal close packing and cubic close packing?
(b)
Crystal lattic and unit cell ?
Ans. (a) Hexagonal close packing follows AB AB AB type arrangement i.e., third layer duplicated first one
and forth layer duplicates second one. In cubic close packing ABC ABC..type arrangement is
present i.e., third layer lie in the depression of second layer that do not lie directly over the
atom of first layer.
(b)
Regular three dimensional arrangement of points in space is known as f space lattic when
these points are replaced with actual atoms or ions it known as crystal lattic. On the other
hand smallest portion of the small lattice which can generate the complete lattic by
repeating own dimensions in various directions is known as unit cell.
Q. Explain :
(a)
The basis of similarities and differences between metallic and ionic crystals.
(b)
Ionic solids are hard and brittle.
Ans. (a) The basis of similarities between metallic and crystal is their regular type of arrangement i.e., ccp,
hcp or bcc arrangement. The basis of difference between these two type of crystal is the
type of ions. In ionic crystals both type of ions positive and negative of different sizes are
present on the specific lattic points while in metallic crystals only positive ions of same size
are packed together and are held together by the negatively charged electrons.
(b)
The hardness of ionic crystals is due to the strong attractivee forces between oppositely
charged ions which keep them in their allotted position. The reason for the brittleness of
ionic crystal is the movement of one layer on the other layer of crystal by the application of
external force. When like ions come infront of each other, the repulsive forces come into
existence and break the crystal.
Q. In terms of band theory, what is the difference
(a)
between a conducutor and an insulator
(b)
between a conductor and a semiconductor.
Ans. (a)
In conductors the energy bands are very close or even overlapped by the conduction band.
Due to this reason electrons can flow very easily from valency band to conduction band under
andelectric field, there by showing conductivity. In an insulator the gap between the valence
band and conduction band is too high that electron cannot flow one to another therefore, no
conduction is possible.
(b)
In semiconductor the gap between valence band and conduction band is small which make
some of the electrons enable to jump from valence band to conduction band therfore,
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conduction is observed. With the increase in temperature, more electrons jump from
valence band to conduction band, hence the electrical conductivity of semiconductor
increases with the increase in temperature.
A cubic solid is made of two elements P and Q. Atoms Q are at the corners of the cube and P at the
body centre. What is the formula of the compound? What are the coordination numbers of P and Q?
The given cubic solid belongs to the body-centered cubic lattice. Since, each corner atom Q is shared
by 8 cubes, hence
No. of Q atoms per cube = 8 × 1/8 = 1
If NaCl is dopped with 10–3 mole % of SrCl2. What is the concentration of cation vacancies?
NaCl is doped with 10–3 mole % of SrCl2 or 100 mole of NaCl ar doped with 10–3 mole of SrCl2or 1
mole of NaCl is doped with 10–3/00 mole of SrCl2
= 10–5 mole of SrCl2
Sr2+ ion will replace 2Na+ ion from lattic hence create one cationic vacancy, therefore,
the conentration of cationic vacancies = 10–5 mol/ mol of NaCl
= 10–5 × 6.02 × 1023 mol–1
= 6.02 × 1018 mol–1
Silver crystallises in fcc lattice. If edge length of the cell is 4.077 × 10–8 cm and density is 10.5 g cm–3,
calculate the atomic mass of silver.
We know that Atomic mass,
M = d × a3 × N gmol–1 Z
Given : Edge length, a = 4.077 × 10–8 Thus, a3 = (4.077)3 × 10–24
cm Density d = 10.5 g cm–3
The number of atoms in fcc unit cell, Z = 4
Thus,
M = 10.5x (4.077)3 x 10–24 6.02x 1023
4
= 107.089 U
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Frenkel defect is not found in pure alkali metal halide. Why?
In alkyl halides Frenkel defect is not found because ions of alkyl halides are bigger than the size
of voids.
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