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  • (a) What type of stiochiometric defect is shown by NaCl and why? (b) Calculate its efficency of packing in case of metal crystal for face centred cubic unit cell.     &

(a) What type of stiochiometric defect is shown by NaCl and why?

(b) Calculate its efficency of packing in case of metal crystal for face centred cubic unit cell.

 

 

 

 
 
 
 
 

Answers (1)

(a) NaCl sshows Schottky defect due to similar size of Na^{+} and Cl^{-}.

(b) Atoms present in face centred cubic unit cell is 4.

Edge length of unit cell (a) = 2\sqrt{2}, where r = radius of atom.

Packing efficency = \frac{vol. of four spheres}{Vol. of unit cell}\times 100

                            = \frac{4\times \frac{4}{3}\pi r^{3}}{(2\sqrt{2}r)^{3}}\times 100

                           = \frac{ \frac{16}{3}\pi r^{3}}{16\sqrt{2}r^{3}}\times 100

                          =\frac{\pi}{3\sqrt{2}}\times 100=74%

a^{2}+a^{2}=(4r)^{2}

2a^{2}=16r^{2}

a^{2}=8r^{2}

a=\sqrt{8r^{2}}=2\sqrt{2}r

Posted by

Sumit Saini

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