What is the co-ordination number of sodium in Na_2 O ?

$\mathrm{Na}_2 \mathrm{O}$ has antifluorite structure. $\mathrm{O}^{2-}$ ions constitute a ccp lattice and $\mathrm{Na}^{+}$ions occupy the tetrahedral voids, each $\mathrm{O}^{2-}$ ion is in contact with $8 \mathrm{Na}^{+}$ ions and each $\mathrm{Na}^{+}$ion is in contact with $4 \mathrm{O}^{2-}$ ions. Thus C.N. of sodium in $\mathrm{Na}_2 \mathrm{O}=4$.