Crystal Structure: in FCC the location of the largest interstitial site, called

Question

Crystal Structure: in FCC the location of the largest interstitial site, called is indicated by the diagram below. On what family of planes are the octahedral sites located (shown above)? How many such sites exist to the FCC unit cell? What are the point coordinates for each of the interstitial sites? How many (whole) impurity atoms per unit cell could potentially occupy the cell's octahedral sites? What is the coordination number for on impurity atom occupying the octahedral site? What size impurity atom could fit there (without straining the lattice) if the host were sodium Give two examples of impurity atoms that could fit.

Explanation / Answer

d) The co-ordination number of impurity at the OCtahedral site is 6.

e) 2R+2r = a, ......... eq-1

now, sqrt(2)*a = 4R .................. eq-2

from eq-1 and eq-2,

(r/R) = 0.414

radius of sodium atom is 227 pm.

thus, r = 227*0.414

or, r = 93.98 or 94 pm.

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