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JEE Main levelSingle correct optionDifficulty 2 of 3ss171

$Na X_{2} O$ crystallises in the antifluorite structure. Which statement describes it correctly?

A $Na X^{+}$ ions form a ccp lattice, $O X^{2 -}$ ions fill all the tetrahedral voids; coordination numbers are 4 ( $Na X^{+}$ ) and 8 ( $O X^{2 -}$ )
B $O X^{2 -}$ ions form a ccp lattice, $Na X^{+}$ ions fill half the tetrahedral voids; coordination numbers are 4 and 4
C $O X^{2 -}$ ions form a ccp lattice, $Na X^{+}$ ions fill all the tetrahedral voids; coordination numbers are 4 ( $Na X^{+}$ ) and 8 ( $O X^{2 -}$ )
D $O X^{2 -}$ ions form a ccp lattice, $Na X^{+}$ ions fill all the octahedral voids; coordination numbers are 6 and 6

Answer: C

Antifluorite is the reverse of fluorite: the anion (

O X^{2 -}

) is in the ccp array with 4 ions per cell, and 8 tetrahedral voids hold 8

Na X^{+}

, giving

Na X_{2} O

. Each

Na X^{+}

has 4

O X^{2 -}

neighbours and each

O X^{2 -}

has 8

Na X^{+}

neighbours.

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