The rotation axis through the two (orange) Fe-IIb atoms is a 4 fold rotation axis, which implies that this rotation axis is the z-axis of PAS and eta = 0 (Theorem 1).
There are also two 2 fold rotation axes, going through two opposite (green) Fe-IIa atoms. This implies that these two 2 fold rotation axes can be chosen as the z-axis of PAS (Theorem 1).
There are also four more 4 fold rotation axes, which go through two opposite (red) Fe-I atoms on the diagonal of the cube. This implies that the EFG tensor is zero as well (Theorem 2). However, this also implies that these rotation axes can be the z-axis of PAS and that eta = 0 (Theorem 1).