  Phase diagrams play a primary role in the understanding of materials
properties. For iron-based superconductors (Fe-SC), the correct definition of
their phase diagrams is crucial because of the close interplay between their
crystallo-chemical and magnetic properties, on one side, and the possible
coexistence of magnetism and superconductivity, on the other. The two most
difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin
of the structural transformation taking place during cooling and its
relationship with magnetism; 2) the correct description of the region where a
crossover between the magnetic and superconducting electronic ground states
takes place. Hence a proper and accurate definition of the structural, magnetic
and electronic phase boundaries provides an extremely powerful tool for
material scientists. For this reason, an exact definition of the thermodynamic
phase fields characterizing the different structural and physical properties
involved is needed, although it is not easy to obtain in many cases. Moreover,
physical properties can often be strongly dependent on the occurrence of
micro-structural and other local-scale features (lattice micro-strain, chemical
fluctuations, domain walls, grain boundaries, defects), which, as a rule, are
not described in a structural phase diagram. In this review, we critically
summarize the results for the most studied 11-, 122- and 1111-type compound
systems, providing a correlation between experimental evidence and theory.
