April 07, 2012

Solid State - II Five Marks

1. Explain Bragg's spectrometer method (in / for crystal study / the study of crystals).
1. Bragg's spectrometer method is one of the important methods for studying crystals using X-rays.
2. The apparatus consists of a X-ray tube from which a narrow beam of X-rays is allowed to fall on the crystal mounted on a rotating table. The rotating table is provided with scale and vernier, from which the angle of incidence, θ can be measured. An arm which is rotating about the same axis as the crystal table carries an ionisation chamber.
3.

X-ray spectrometer
4. The rays reflected from the crystal enter into the ionization chamber and ionise the gas present inside.
5. Due to the ionisation, current is produced which is measured by electrometer.
6. The current of ionisation is a direct measure of intensity of reflected beam from the crystal. For different angles of incidence, the corresponding ionisation current is measured from the electrometer.
7. These values are plotted in the form of graph.
8. For sodium chloride, the maximum reflection or peaks for 100 plane occurred at θ = 5.9°, 11.85° and 18.15°. The sines of these angles are 0.103, 0.205 and 0.312 which are in the ratio 1 : 2 : 3. These peaks represent the first,second and third order reflections.
The ratio confirms the correctness of Bragg’s equation.
2. Explain Schottky defect and Frenkel defect (in crystals).
1. Schottky defect is caused if some of the lattice points are unoccupied. The points which are unoccupied are called lattice vacancies.
The number of missing positive and negative ions is the same in this case and thus, the crystal remains neutral.
The existence of two vacancies, one due to a missing Na+ ion and the other due to a missing Cl- ion in a crystal of NaCl is shown in Fig

Schottky Defects in NaCl Crystal
A Schottky defect appears generally in ionic crystals in which the positive and negative ions do not differ much in size.

A few examples of ionic compounds exhibiting Schottky defect are NaCl, KCl, KBr, CsCl etc.,
2. Frenkel defect arise when an ion occupies an interstitial position between the lattice points.
This defect occurs generally in ionic crystals in which the anion is much larger in size than the cation.
AgBr is an example for this type of defect.
One of the Ag+ ion occupies a position in the interstitial space rather than its own appropriate site in the lattice is shown in Fig
Frenkel Defects in AgBr Crystal
The crystal remains neutral since the number of positive ions is the same as the number of negative ions.
A few examples of ionic compounds exhibiting Frenkel defect are AgCl, AgBr, AgI, ZnS etc.,
Or
S.
No
Schottky defect
Frenkel defect
1
Caused if some of the lattice
points are unoccupied. 
(lattice vacancies)
Arise when an ion occupies an
interstitial position between
the lattice points.
2
Appears generally in ionic crystals
in which the positive and negative
ions do not differ much in size.
Occurs generally in ionic crystals
in which the anion is much
larger in size than the cation.
3
Crystal remains neutral since the
number of missing positive and negative ions is the same.
Crystal remains neutral since
the number of positive ions
is the same as the number of
negative ions.
4
Example: NaCl, KCl, KBr, CsCl

Example: AgCl, AgBr, AgI, ZnS
5

3. Explain the nature of glass.
1. The chief characteristics of a glass are hardness, rigidity and ability to withstand shearing stresses which are all properties of the solid state.
2. On the other hand glasses are optically isotropic and on heating without any sharp transition passes into a mobile liquid.
3. At a high temperature glasses undergo phase transition when crystals separate first as they do form super cooled liquid. Therefore, glasses are regarded as amorphous solids or super cooled liquids as well.
4. Thus, glassy or vitreous state is a condition in which certain substance can exist, lying between the solid and liquid states.
4. What are superconductors? Write their uses.
Super conductors
The ability of certain ultra cold substances to conduct electricity without resistance is called super conductivity. This superconductivity state is a state in which a material has virtually zero electrical resistance. Substances having this property are called super conductors.
Uses of superconductors
1. It is a basis of new generation of energy saving power systems. Super conducting generators are smaller in size and weight when we compare with conventional generators. These generators consume very low energy and so we can save more energy.
2. High efficiency ore separating machines may be built using superconducting magnets.
3. Superconducting solenoids are used in Nuclear Magnetic Resonance Imaging equipment which is a whole body scan equipment.
5. Write notes on any two point defects in Crystals.
If the deviation occurs due to missing atoms, displaced atoms or extra atoms, the imperfection is named as a point defect.
Such defects arise due to imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures.
The most common point defects are the Schottky defect and Frenkel defect. Comparatively less common point defects are the metal excess defect and the metal deficiency defect.
1. Metal excess defects
If 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 NaCl in which there is a slight excess of sodium ions. This defect is called the metal excess defect.
2. Metal deficiency defects
In certain cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring additional charges instead of original charge. This type of defect is generally found in compounds of transition metals which can exhibit variable valency.
FeO and FeS show this type of defects.
6. Write the properties of ionic crystals.
1. The heats of vapourisation of ionic crystals are high.
2. The vapour pressure of ionic crystals at ordinary temperature are very low.
3. The melting and boiling points of ionic crystals are very high.
4. Ionic crystals are hard and brittle.
5. Ionic crystals are insulators in the solid state.
6. Ionic crystals are soluble in water and also in other polar solvents.
7. Ionic solids are good conductors when dissolved in water
ONE MARKS  THREE MARKS

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