On the interpretation of phase diagram of La2−x(Ba,Sr)x;CuO4

It is shown that the features of La_2–x(Ba,Sr)_xCuO₄ around x=0.125 can be. treated as a consequence of alternate-layer doping when doped holes are added to each second CuO₂ plane. The model of heterovalence doping of high-T_c superconductors (HTSC) is proposed to account for the evolution of HTSC properties with doping. According to this model the transition from insulator to metal state under doping proceeds through the stage of-U-centers formation, which arise when the definite spatial arrangement of the doping atoms is realized.