The axis connecting Fe-IIa and N is a twofold rotation axis, similar to the axes connecting Fe-IIa atoms with Fe-I atoms. The axes connecting Fe-IIa with other Fe-IIa atoms and Fe-IIb do not have a special rotational symmetry.
The axis connecting Fe-I with Fe-IIb atoms is also a twofold rotation axis. However the most important rotation axis is the one connecting Fe-I atoms with other Fe-I atoms, these axes have a four-fold rotational symmetry. Therefore this is the z-axis of the principle axis system, and the electric field gradient tensor will be zero around this symmetry axis.