  The mechanism of -U center formation in high-$T_{c}$ superconductors (HTS)
with doping is considered. It is shown that the transition of HTS from
insulator to metal passes through the particular dopant concentration range
where the local transfer of singlet electron pairs from oxygen ions to pairs of
neighboring cations (-U centers) are allowed while the single-electron
transitions are still forbidden. We believe it is this concentration range that
corresponds to the region of high-Tc superconductivity and the interelectron
attraction results from the interaction of electron pairs with -U centers.
Additional hole carriers are generated as the result of singlet electron pair
transitions from oxygen ions to -U-centers. The orbitals of the arising singlet
hole pairs are localized in the nearest vicinity of -U center. In such a system
the hole conductivity starts up at the dopant concentration exceeding the
classical 2D-percolation threshold for singlet hole pair orbitals. In the
framework of the proposed model the phase diagram Ln-214 HTS compounds is
constructed. The remarkable accord between calculated and experimental phase
diagrams may be considered as the confirmation of the supposed model. The main
features of hole carriers in HTS normal state are found to be the nondegenerate
distribution and the dominant contribution of electron-electron scattering to
the hole carrier relaxation processes. Various experimentally observed
anomalies of HTS properties are shown to be the consequences of the
above-mentioned features. The conclusion is made that HTS compounds are the
special class of solids where the unusual mechanism of superconductivity
different from BCS is realized.
