Hole-doping of the CuO2 planes in high Tc superconductors

The concept of ‘net holes’ as calculated from the existing crystallographic data via the bond-valence-sum method is utilized for discussing the charge transfer balances in layered cuprates. When focusing on the net holes, i.e. summing the excess positive charge residing either on copper or on oxygen in the CuO2 planes, one notices that in the resulting p(CuO2) parameter all the contributions from the planar Cu–Opl bonds are counteracted and only the vertical bonds from Cu to the apical oxygen (Oapi) and from Opl to the nearest-neighbouring cations have net effects on the hole concentration. Based on this observation, three different ways of doping the CuO2 planes are defined and illustrative examples of each doping route are given. Furthermore, in order to estimate the distribution of holes between inequivalent CuO2 planes in the n≥3 members of homologous series of superconducting cuprates, expressed by MmA2Qn−1CunOm+2+2n±δ or M-m2(n−1)n, the p(CuO2)’s for the individual CuO2 planes were calculated. As a surprising result, the holes were found to be confined in the innermost CuO2 planes in all the systems analyzed. This observation is discussed in comparison with the results obtained for n-type and p-type doped infinite-layer compounds. Finally, the roles of the apical anion and the Cu–O bond length in the CuO2 plane are considered.