Sorry, I didn’t motice the Fe IIb atom in the previous answer.
Fe-I, there are 3 4-fold rotation axes. 3 on x, y and z direction. Each of the 4-fold axes can be chosen as the z-axil of the PAS and makes η=0. Since there’re more than 2 4-fold axes in each case, the EFG tensor is zero.
For Fe-IIa, there are 1 4-fold roataion axe on the z direction and 2 2-fold axes on the x and y direction. The 4-fold axe can be chosen as the z-axil of the PAS and makes η=0.
For Fe-IIb, there are 1 4-fold roataion axe on the z direction and 4 2-fold axes on the x and y direction and within the plane of the atom. The 4-fold axes can be chosen as the z-axil of the PAS and makes η=0.