  It is demonstrated qualitatively how a unified explanation of the wide
variety of different regions in a typical phase diagram of HTSC can be given in
terms of the model of negative-U centers superconductivity. Both the existence
of four regions (pseudogap, superconducting, non Fermi-liquid and
Fermi-liquid), in the phase diagram and all the transitions between these
regions were explained by assuming that negative-U centers exist in HTSC and
qualitatively taking into account their thermodynamic and direct
quantum-mechanical interaction with ordinary electrons from the valence band.
It is found that the principal parameter determining the properties of HTSC in
the model of negative-U centers is the relative concentration of electrons
belonging to negative-U centers. The negative-U centers naturally determine
both the superconducting properties of HTSC systems and the entire phase
diagram in the normal state.
