Question

What is the location of voids in a crystal?

Answer 1

As we know spheres can't fill the entire space i.e. packaging factor <1 for all crystals. This implies that there are voids between the atoms.These voids have complicated shapes but we are mostly interested in the largest sphere which can fit into these voids hence plane faced polyhedron version of the voids is only considered.The position of a void of a particular type will be consistent with the symmetry of the crystal.