(i) What type of non - stoichiometric point defect is responsible for the pink color of $L i C l$ ?
(ii) What type of stoichiometric defect is shown by $N a C l$ ?
OR
How will you distinguish between the following pairs of terms:
(i) Tetrahedral and octahedral voids
(ii) Crystal lattice and unit cell

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Solution

$\to D u e t o m e t a l e x c e s s d e f e c t, a n i o n I C v a c a n c i e s a r e f o r m e d i n L i C l w h i c h a r e r e s p o n s i b l e f o r i t s p i n k c o l o u r . \to N a C l s h o w s s c h o t t l y d e f e c t (s t o i c h i o m e t r i c d e f e c t) i n t h i s, e q u a l n o . o f c a t i o n s a n d a n i o n s a r e m i s s i n g f r o m t h e r e l a t t i c e O R \Rightarrow T e t r a h e d r a l (a) i t i s a v o i d w h i c h i s s u r r o u n d e d b y 4 s p h e r e s . i . e c o o r d i n a t i o n n o . = 4 (b) t h e y a r e o b s e r v e d o n e d g e s o f t h e u n i t c e l l .$
$\Rightarrow O c t a h e d r a l (a) i t i s a v o i d w h i c h i s s u r r o u n d e d b y 6 s p h e r e s i . e . c o o r d i n a t i o n n o . = 6 (b) t h e y a r e o b s e r v e d i n t h e c e n t e r o f t h e u n i t c e l l .$
$\Rightarrow P h y s i c a l L a t t i c e (a) i t i s a a r r a n g e m e n t o f p o i n t i n s p h e r e (3 - g) (b) l a t t i c e a r e c o m p l e x a n d l a r g e$
$\Rightarrow U n i t c e l l (a) i t i s a s m a l l e s t p a r t o f a c r y s t a l l a t t i c e w h i c h w h e n r e p e a t e d i n d i f f e r e n t d i r e c t i o n f o r m t h e e n t i r e c r y s t a l l a t t i c e (b) u n i t c e l l a r e s i m p l e a n d s m a l l e r .$

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