For the face-centered iron particles:
We can identify a four-fold rotation axis if we take the axis starting at the center nitrogen and go towards the face-centered iron. Furthermore, one can identify two two-fold rotation axes perpendicular to the four-fold axis. Thus we take the four-fold axis as z-axis of our PAS and can know that eta is 0. And we can complete the PAS with the two-fold axes.
For the corner iron particles:
The yellow arrow already indicates a four-fold rotation axis. There are 2 more four-fold rotation axes perpendicular to the first one, thus our system experiences a EFG of 0.