Solid State - Result Question 15

####15. CsCl crystallises in body centred cubic lattice. If ’ a ’ its edge length, then which of the following expressions is correct?

(a) rCs++rCl=3a

(b) rCs++rCl=3a2

(c) rCs++rCl=32a

(d) rCs++rCl=3a

(2014 Main)

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Answer:

Correct Answer: 15. (c)

Solution:

  1. In CsCl,Cllies at corners of simple cube and Cs+at the body centre. Hence, along the body diagonal, Cs+and Cltouch each other so rCs++rCl=2r

Calculation of r

In EDF,

Body centred cubic unit cell

In AFD,

FD=b=a2+a2=2a

c2=a2+b2=a2+(2a)2=a2+2a2 c2=3a2c=3a

As AFD is an equilateral triangle.

3a=4r r=3a4  Hence, rCs++rCl=2r=2×34a=32a



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