Explanation
$$FeO$$ has metal deficiency defect. Metal deficiency defect is supposed to arise when there are lesser number of positive ions than negative ions. In case of $$FeO$$, the positive ions are missing from their lattice sites.
The additional negative charge is balanced by some nearby metal ion by acquiring one more positive charge. It happens in $$FeO$$ because $$Fe$$ has capacity of showing variable oxidation states.
Since, octahedral voids are present at body center and edge centers. So, if $$h$$ is the height of HCP unit cell, the body center must be located at $$\cfrac h2$$ height from the base, where $$a$$ octahedral void is located.
Gustav F. Hüttig first proposed the radius ratio rule in 1920.
The table below gives the relation between radius ratio, coordination number and type of structure.
Radius Ratio Coordination number Type of structure
< 0.155 2 Linear
0.155 - 0.225 3 Triangular Planar
0.225 - 0.414 4 Tetrahedral
0.414 - 0.732 6 Octahedral
0.732 - 1.000 8 Cubic
The correct option is C.
In face centered cubic unit cell (FCC), tetrahedral voids are present on body diagonals. There are two tetrahedral voids on each body diagonal at $$\left (\cfrac 14 \right)^{th}$$ of the distance from each corner.
So, total number of tetrahedral voids = (no. of tetrahedral voids per body diagonal) x (no. of body diagonals) = $$2 \times 4 = 8$$
Hence, Option "B" is the correct answer.
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