CuO chain statistics, charge transfer and Tc(x) dependence in   YBa2Cu3O6+x, Y0.9(Ca)0.1Ba2Cu3O6+x, and Y0.8(Ca)0.2Ba2Cu3O6+x
  x dependences of Tc in YBa2Cu3O6+x and Y1-b(Ca)bBa2Cu3O6+x (b=0.1 and b=0.2) have been calculated assuming that the net doping of CuO2 layers is a sum of contributions from CuO chains and from substitution of Y3+ by Ca2+. We applied the concept of minimal (critical) chain length lcr needed to trigger charge transfer from chains to planes. The model proposed assumes that only a certain part, say kappa, of those chain-holes that are created beyond the first lcr-2 holes in chains of length l>lcr, are able to attract electrons from CuO2 bilayer. Our analysis points to the conclusion that parameter lcr should be equal to 4 (four oxygen atoms in a chain), or very close to it (3, or 5). Calculated x dependences of doping, p(x), at constant (room) temperature and for three different substitution levels b=0, 0.1, and 0.2, are found to be in excellent agreement with available experimental data. These p(x) dependences are combined with universal (parabolic) phase relation Tc(p) to obtain three Tc(x) dependences that also remarkably correlate with those reported in experiments. The results obtained indicate that in long chains (x=1) the probability for a chain-hole to capture an electron (expressing hole ability to become transferred) decreases with the concentration of 3d Cu electrons in CuO2 layers, ranking from kappa=40%(42%) in YBa2Cu3O6+x, over kappa=36% in Y0.9(Ca)0.1Ba2Cu3O6+x to kappa=33% in Y0.8(Ca)0.2Ba2Cu3O6+x. We estimate that in these three systems the wavelength of charge corrugations in long chains (at x=1) should be ranking around lambda=1.38nm, lambda=1.25nm, lambda=1.20nm, respectively. 
V. M. Matic