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I always wonder that though Both $\ce{NaCl}$ and $\ce{CsCl}$ are alkali halides, why they adopt different crystal forms, FCC and BCC respectively? and I have a doubt about $\ce{CsCl}$ structure that Why the $\ce{Cs+}$ or $\ce{Cl-}$ ions are not located at voids/lattice points as like as $\ce{NaCl}$ structure?

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Perhaps you have heard of the following terminology: "rock salt", "cesium chloride", "zinc blende" structures? If so, you may recall that in this case, the main factor dictating structure is "size" or atomic radius ratio, rather than group. For example, in the case of the rock salt structure, it is most likely that there is "a cation/anion radius ratio of 0.414 to 0.732". As such, you could consider the radius of cesium in comparison to sodium: significantly larger. Thus, it adopts the cesium chloride configuration, which is preferred when "ions are of roughly the same size ".

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