## Question

A binary solid X^{+} Y^{-} has a zinc blende structure with Y^{-} ions forming the lattice and X^{+} ion occupying 25% tetrahedral sites. The formula of the solid is

### Solution

XY_{2}

In zinc blende, Y^{-} ions form ccp lattice and have two tetrahedral sits per atom or sphere.

For the formula XY, there are four tetrahedral sites for two atoms. Thus, the 25% of tetrahedral site, *ie*, one tetrahedral site is occupied by X^{+}. Therefore, the formula of the compound is XY_{2}.

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