The doping routes and distribution of holes in layered cuprates: a novel bond-valence approach

In the present contribution a general view on the charge balance and doping in superconducting and related layered cuprates is developed. For a quantitative discussion the concept of bond valence as formulated by Brown and Altermatt is utilized. When summing up the excess positive charge above +2 for Cu and -2 for the in-plane O in one CuO2 unit, that is focusing on the 'net holes' shared by Cu and O in the CuO2 planes, all the contributions from the in-plane Cu-O bonds are counteracted and only the bonds from Cu to the apical O and from the inplane O to the nearest-neighbour cations in the layers above and below the CuO2 plane have net effects on the resulting hole concentration parameter p(CuO2). From this observation, three distinct ways of hole doping the CuO2 planes are defined. Representative examples of each doping route are discussed in terms of bond-valence analysis based on existing crystallographic data. Furthermore, bond-valence calculations were utilized to estimate the distribution of holes among the inequivalent CuO2 planes in the members with n greater than or equal to 3 of various homologous series of superconducting cuprates, expressed as M(m)A(2)Q(n-1) CunOm+2+2n+/-delta or M-m2(n-1)n. As a surprising result, the holes were found to be confined in the innermost CuO2 planes in all the n greater than or equal to 3 phases analysed. In each homologous series the optimized superconducting transition temperature T-c,T- opt is always the highest for the n = 3 member and shows less variation among the different n greater than or equal to 3 phases than in the cases of n = 1 or 2. The bond-valence calculation result as well as these empirical observations could be explained in a most straightforward way by supposing that the inner square-planar CuO2 planes would predominantly determine the value of T-c in the n 3 phases. Proceeding along this interpretation, it seems that in terms of obtaining the universally highest T-c the hole concentration in the innermost CuO2 plane, the hole distribution in the in-plane Cu-O bond and the degree of hole confinement (both intraplane and interplane) are the parameters to be optimized. On the other hand, for improving the irreversibility-field (H-irr) property, homogeneity in the hole distribution over the crystal unit along the piling direction of the various layers is important. For 'on-demand' tailoring of the cuprate materials, that is simultaneous controlling of T-c and H-irr, to master the ways to guide the holes in the desired parts of the layered structure, that is to understand the distinct effects of the three hole-doping routes, is the key point.