A stepwise dependence of the critical temperature on oxygen contents and isotopic effect anomalies in Y Ba2 Cu3O6+x resulting from the plasmon mechanism of high-Tc

It is shown that the nonmonotonic (stepwise) character of the dependence of the superconducting transition temperature Tc upon x observed in Y Ba2 Cu3 O6+x, along with the anomalously weak negative isotopic effect versus oxygen in Y Ba2 Cu3 O7 on selective substitution of 16 O with 18 O in the Cu O2 plane, is the result of competition of the plasmon and phonon HTSC mechanisms. The final isotopic effect versus copper is predicted for Y Ba2 Cu3 O6+x at 0.50 ≤ x ≤ 0.75 and for other metal-oxide cuprate compounds with Tc < 60 K