  This paper reviews experimental phase diagrams of cuprates and pnictides to
demonstrate that specific features of the superconducting phase diagrams in
bothHTSC families can be understood within the framework of the proposed
approach,which assumes the formation, under heterovalent doping, of localized
trion complexes consisting of a doped carrier and charge transfer (CT)
excitons. The geometry of such cells containing CT excitons (CT plaquettes) in
the basal plane of the crystal is determined by its crystal structure and the
type of dopant, so that the dopant concentration range corresponding to the
existence of a percolation cluster of CT plaquettes can be readily determined
for each particular compound. These dopant concentration ranges coincide with
good accuracy with the experimental ranges of superconducting domes in the
phase diagrams of the HTSC compounds considered. The generation of free
carriers and the mechanism of superconducting pairing in this pattern is
related to biexciton complexes (Heitler-London centers) emerging in neighboring
CT plaquettes.

Superconductivity