Evaluation of CuO2 plane hole doping in YBa2Cu3O6+x single crystals

The electron hole concentration p in the CuO2 planes of YBa2Cu3O6+x (YBCO) has been evaluated using the unit cell length in the c direction together with a simple estimation for p in the chain oxygen-ordered phases. The empirical relationship between p and c obtained allows the calculation of p from the c-axis lattice parameter for the entire doping range of YBCO. It is also suggested that the empirical parabola describing the relationship between T-c and p in La2-xSrxCuO4 is reasonably correct for YBCO except for the region around p=1/8 (x=0.67), where the T-c of YBCO is suppressed by as much as 17 K. A plot of p vs x and a correction of the parabola for the p=1/8 effect are presented, allowing convenient determination of p from x and T-c.