In terms of band theory, what is the difference between a conductor and an insulator?
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132.
In terms of band theory, what is the difference between a conductor and a semi-conductor?
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133.Explain the following terms with suitable examples: 'Schottky defect'
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134.Explain the following term with suitable examples: 'Frenkel defect'
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135.Explain the following term with suitable examples: 'Interstitials'.
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136.Explain the following term with suitable examples: 'F-centres'.
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137.Examine the illustration of a portion of the defective crystal given below and answer the following questions: (i) What are these types of vacancy defects called? (ii) How is the density of a crystal affected by these defects? (iii) Name one ionic compound which can show this type of defect in the crystalline state. (iv) How is the stoichiometry of the compound effected?
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138.Schottky defects generate an equal number of cation and anion vacancies while doping produces only cation vacancies and not anion vacancies. Why?
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139.The ions of NaF and MgO all have the same number of electrons, and the internuclear distance are about the same (235 pm and 215 pm). Why then are the melting points of NaF and MgO so different. (992°C and 2642°C)?
140.How would you account for the following: Frenkel defects are not found in alkali metal halides.
The frenkel defect is favoured by a large diffrance in size between the positive and negtive ion the metal cation are generally smaller than the anion which occupy the vacant lattice site or hole.
Frenkel defects are not found in alkali metal halides because the ions cannot get into interstitial positions due to their large sizes.
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