Chemical insights obtained by modelling the structure of high-temperature superconductors using AX3 close-packing and transferable single atom parameters

The structure of the high-temperature AxCunOy superconductors have been analyzed in terms of admixtures of two kinds of close-packed AO3 which are distinguished by the relative sequence of AO and OO rows. Such layers generate the superconductors when arranged in cubic close-packing sequence with oxygen vacancies. By assigning pseudo-spins to the two basic kinds of packing and applying the uniaxial magnetic interaction model we find that three kinds of A ions may be associated with three kinds of pseudo-spin sites. In the superconductors, these sites correspond to what we term fluorite-like layers, rock-salt-like layers and domain boundaries. The various A ions may be distinguished by their single atom transferable parameters such as the empirical coordination number dependent ionic radii or the non-local angular-momentum dependent pseudo- potential orbital radii. Such an analysis helps to place all the copper oxide superconductors in one structural framework and accounts for the various oxygen intercalation processes. It also helps in defining an average tolerance factor which emphasizes in a sense the elastic effects on the lattice due to different sizes of the A ions. The superconductors with Tc > 60 K are characterized by the presence of at least one pure ''rock-salt'' layer as well as one ''fluorite'' layer. The importance of the rock-salt layer for the crystallographic and superconducting properties is discussed. Analysis of the tolerance factor as well as thermodynamic data for individual oxides indicates that a valence fluctuation in the rock-salt layer could be coupled to valence fluctuations in the CuO2 planes. It is suggested that such a coupling helps in enhancing the superconducting transition temperatures. (orig.)