Why do $\ce{NaHCO3}$ and $\ce{KHCO3}$ have different crystal structures?

We had this in a test today and this got me stumped...

My questions are:

  • Do $\ce{NaHCO3}$ and $\ce{KHCO3}$ possess crystal structures?
  • If they do, why should they have different crystal structures, because they are in the same group and must possess almost similar physical properties.

The reason why the crystal structure of $\ce{NaHCO3}$ is face centered and the crystal structure of $\ce{KHCO3}$ is body centered is due to the fact that the cation/anion ratio is different, meaning that certain types of hole are now too small for the cation to fit into them and as a result it must go into a different hole.


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