For Fe-I there are three (x, y, z directions) 4-fold rotation axes, which means that the EFG tensor is 0.
For Fe-II there is one 4-fold rotation axis in the z direction and 2 2-fold rotation axes in the x, y directions. The z axis (passes through Fe-IIb and N) can be taken as the z-axis of the PAS. η=0.