In all superconducting Fe-based oxy-pnictides and chalcogenides, the common structural feature is a layer of PbO-like corner sharing tetrahedra. The distortion of such tetrahedra plays a key role in tuning the electronic properties and the ground state. The structural simplicity of FeCh (Ch=S,Se,Te) offers the best tool for investigating the nature of superconductivity and magnetism in these class of compounds. When partially substituting Se for Te in the antiferromagnetic Fe1+xTe, the excess of Fe is reduced and superconductivity appears over a wide range of compositions. Both the excess Fe and the Se substitution affect the structure and must be kept under control for tuning the electronic properties, thus drawing a three-dimensional T-composition phase diagram. The crossover from one antiferromagnetic order to another, and therefore to a superconducting state at low T, takes place at a critical distance between the Fe site and the Te(Se) plane. Pressure does not act as che

mical substitutions in this system, being the effect of the latter on the FeCh4 tetrahedron strongly anisotropic. In the low Se-content region, local composition fluctuations, phase separation, disordered split position of elements, and spin-glass behavior, lead to a very complex scenario in which the coexistence of superconductivity and magnetic order is claimed. The study of how the antiferromagnetic state is left and superconductivity occurs is still under investigation by various experimental techniques. In order to shed light on this point and unambiguously understand the behavior of this material at the verge between superconductivity and magnetism, a high level of purity and crystalline quality of samples is mandatory.