A model for the charge transfer mechanism in YBa2Cu3O6+x high-Tc cuprate based on the critical chain length concept is proposed to account for the 60 and 90 K plateaus in the Tc(x) dependence. It has been shown that, when the statistics of CuO chain formation was described in terms of the two-dimensional asymmetric next-to-nearest neighbor Ising (ASYNNNI) model, at any constant temperature below the top of ortho-II phase there exists a uniquely defined value of critical chain length lcr(T) that yields a constant doping p(x)≈const over the regime of ortho-II phase (related to the 60 K plateau of Tc(x)), while the 90 K plateau coincides with the monotonically increasing p(x) over the optimal doping level p = 0.16 in the regime of the ortho-I phase. Short length chains (l

A model of charge transfer mechanism from CuO chains to CuO 2 planes has been proposed to account for doping of the planes, assuming that only chains containing more than three oxygen atoms can contribute to hole transfer. Only chains with l≥4 are assumed to have transferred a certain fraction, approximately 40%, of the holes created by oxygen atoms added to the chain beyond the first three oxygen atoms. Using the so obtained x dependence of doping, p(x), at constant (room) temperature and utilizing empirical parabolic phase relation T c (p) (T c (p)=T c,max [1-82.6(p(x)-0.16) 2 ], the T c versus x dependence is obtained to have two clearly distinguished plateaus at 60K and 90K, remarkably fitting to experimental T c (x). The effect of statistics of CuO chain fragmentation has been included by applying cluster variation method to two dimensional asymmetric next nearest neighbor Ising model that is employed to describe oxygen-chain ordering in basal planes. The obtained results indicate that plateaus, coinciding with T c '(x)=0, emerge either when p'(x)=0 (p(x)≈const), in the region of OII phase formation (the 60K plateau), or when p=0.16, representing the optimal doping at x≈0.91 in OI phase (the 90K plateau).

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