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Schottky defect also exists in pair to maintain electroneutrality. Therefore, it generates equal number of cation and anion vacancies. In case of doping of NaCl by CdCl 2 , CI – ions enter Cl – ions site in NaCl while Cd 2+ displace Na + . In doping so, one positive charge becomes extra. Therefore one Na + is displaced so as to maintain electro-neutrality. That is doing only cation vacancies exist.

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The Solid State

Short Answer Type

131.

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?

Schottky defect also exists in pair to maintain electroneutrality. Therefore, it generates equal number of cation and anion vacancies. In case of doping of NaCl by CdCl₂, CI^– ions enter Cl^– ions site in NaCl while Cd²⁺ displace Na⁺. In doping so, one positive charge becomes extra. Therefore one Na⁺ is displaced so as to maintain electro-neutrality. That is doing only cation vacancies exist.

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Long Answer Type

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)?

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Short Answer Type

140. How would you account for the following:
Frenkel defects are not found in alkali metal halides.

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