1.

An alloy of metals M and N, crystallises in ccp. Atoms of N occupy lattice points and atoms of M occupy one-third of tetrahedral voids. What is the composition of the unit cell?

Answer»

Solution :Unit CELL of ccp has 8 atoms of N at comers and 6 atoms of N at face centres.
Number of atoms of `N= 8 xx (1)/( 8) +6 xx (1)/(2)=1 + 3=4`
Number of tetrahedral voids `=2 xx 4=8`
Number of atoms of M `=8 xx (1)/(3) = (8)/(3)`
Ratio of number of atoms of M and number of atoms of N `=(8)/(3) : 4 = (8)/(3) : (8)/(2) = 2:3`
Composition of ALLOY is `M_(2) N_(3)`


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