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The CsCl lattice (bcc) is more stable th...

The CsCl lattice (bcc) is more stable than `NaCl(fc c)`. Why do halides of Li, Na, K and Rb not have this lattice.

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Radius ratio effect. The radius ratio in the halides of Li, Na, K and Rb lies between 0.732 and 0.414 and, therefore, the coordination number has to be 6.
If 8 halide ions are packed around a smaller positive ion, say `Na^(+)`, the negative ions will be closer to each other than to the positive ion. The lattice would thus be unstable. This is overcome by packing 6 negative ions around the central positive ion, and hence coordination number 6.
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