Question

What is the coordination number of $C{s}^{+}$ and $C{l}^{-}$ in $CsCl$ structure?

Answer 1

This is obvious if we compare the $CsCl$ unit cell with the simple cubic unit cell showing the interstitial site. We may also consider the neighborhood in which each ion finds itself. Each ${Cs}^{+}$ is surrounded by 8 ${Cl}^{-}$ (so the${Cs}^{+}$coordination number is 8) at the corners of each cube.Each ${Cl}^{-}$ is also surrounded by 8 ${Cs}^{+}$ at the corners of a cube, so the ${Cl}^{-}$ has CN = 8.