  We propose a generalized model of electronic structure modification in HTSC
cuprates and ferropnictides under doping. In this model the role of doping
consists in only a local change in the electronic structures of the parent
phases of cuprates and ferropnictides due to the formation of trion complexes
comprising a doped carrier localized in unit cell and charge transfer (CT)
excitons around it. These CT excitons emerge in CuO4 or AsFe4 plaquettes in the
CuO2 or FeAs basal planes (CT plaquettes) under the influence of doped carrier,
restricting its itinerancy. As the dopant concentration is increased, CT
plaquettes combine into clusters of the so called CT phase. It is this CT phase
that is related in the model to the HTSC phase. In support of this assumption,
we determined the ranges of dopant concentrations conforming to the existence
of percolation clusters of the CT phase; these ranges were shown to coincide
with the positions of the superconducting domes on the phase diagrams of these
compounds. The model also perfectly describes subtle features of the phase
diagrams of various cuprates and ferropnictides including the 1/8 anomaly,
narrow peaks in the dependences of the London penetration depth on the
concentration of the dopant, and other specific features. The mechanism of the
generation of free carriers in the CT phase, provided by intrinsic self-doping,
was considered. The mechanism is not directly related to external doping, but
is due to the interaction of band electrons with so called Heitler-London (HL)
centres inherently existing in the percolation cluster of CT phase and
representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs
basal planes. Material in CT phase was shown to represent a medium, in which
the mechanism of excitonic superconductivity, specified by the interaction of
band electrons with HL centres, can be realized.
