Impact of chain fragmentation on charge transfer scenario and   two-plateaus-like behavior of Tc(x) in YBa2Cu3O6+x
  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). 
V. M. Matic and N. Dj. Lazarov