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(B) In an $NaCl$ crystal structure (fcc),the $Na^{+}$ ion is surrounded by $6$ nearest neighbors $(Cl^-)$ at a distance of $a/2$. The next nearest nei

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$6$

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(B) In an $NaCl$ crystal structure (fcc),the $Na^{+}$ ion is surrounded by $6$ nearest neighbors $(Cl^-)$ at a distance of $a/2$. The next nearest neighbors are $12$ $Na^{+}$ ions at a distance of $a/\sqrt{2}$. Thus,$X = 12$.In a $CsCl$ crystal structure (bcc),the $Cs^{+}$ ion is surrounded by $8$ nearest neighbors $(Cl^-)$ at a distance of $\sqrt{3}a/2$. The next nearest neighbors are $6$ $Cs^{+}$ ions at a distance of $a$. Thus,$Y = 6$.Therefore,$X - Y = 12 - 6 = 6$.

Number of next nearest ions around $Na^{+}$ in $NaCl$ unit cell is $X$ and number of next nearest ions around $Cs^{+}$ in $CsCl$ unit cell is $Y$. The value of $X - Y$ is

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