The Solid State

What is the crystallograph dimension of monoclinic unit cell?
In the given crystal what should be the cation $$X$$ which replaces to create a $${ Na }^{ + }$$ cation vacancy?
For a given anion, the graph for NaCL type structure levels off at low ($$r^+/r^-$$) values because of ?
$$Fe_3O_4$$ is ?
Classification of crystalline solids?