  A model of charge transfer mechanism from CuO chains to CuO2 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 length l greater than 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 Tc(p) (Tc(p)=Tc,max[1-82.6(p(x)-0.16)2], the Tc versus
x dependence is obtained to have two clearly distinguished plateaus at 60K and
90K, remarkably fitting to experimental Tc(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 dTc(x)/dx=0, emerge either when
dp(x)/dx=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).
