Abstract
Convex polyhedral cuprate clusters are being formed through lateral frustration when the a and c lattice parameters of the tetragonal ACuO2 infinite layer structure will become identical by substitution of a large cation (A = Ba2+). However, the corner-shared CuO2 plaquettes of the infinite network suffer a topotactic rearrangement forming edge-connected units, for instance Cu18O24 cages (polyhedron notation [4641238]) with 2 compound (space group P4/ nmm) will be discussed. The possibility to construct a cuprate super-cage with m3m symmetry (polyhedron notation [4641242438]) is being reported. This super-cage still consists of edge-connected CuO2 plaquettes when fully decorated with copper ions, but with different curvatures, arranged in circles of 9.39 ? of diameter with 139.2° Cu-O-Cu antiferromagnetic super-exchange interaction. On the one hand, the realization of such a quite stable cuprate super-cage as a candidate for high-Tc superconductivity depends on whether a template of suitable size such as the cation or C(CH3)4 enables its formation, and on the other hand the cage can further be stabilized by highly charged cations located along the [111] direction. Synthesis options will be proposed based on suggested cage formation pathways. An X-ray powder pattern was calculated for a less dense cluster structure of Im3m space group with a lattice parameter of a = 14.938 ? and two formula units of Cu46O51 to facilitate future identification. Characteristic X-ray scattering features as identification tool were obtained when the electron distribution of the hollow polyhedron was approximated with electron density in a spherical shell.
Highlights
Almost thirty years of intense research on high-Tc superconductors have elapsed this year
The results provide a possible explanation of the astonishing structural differences between the infinite-layer and the cubic BaCuO2 structures
Since the suggested templates lack a centre of symmetry, one could adapt this fact to the cage structure, too, and would create an acentric space group such as I432 with more degrees of freedom to account for energetically favoured bond lengths and angles, respectively
Summary
Almost thirty years of intense research on high-Tc superconductors have elapsed this year. Whereas magnetic properties have been extensively investigated, the synthesis of pure dioptase is not successful yet but may benefit from ideas of cluster formation expressed in this work It came as a great surprise when Sigrist et al [7] succeeded in synthesizing Ca0.86Sr0.14CuO2, a very simple compound of tetragonal symmetry (space group P4/mmm) with lattice parameters of a = 3.8611(2) Å and c = 3.1995(2) Å and Z = 1, which they named parent structure of the layered high-temperature superconductors. Siemons [8] discovered that cupric oxide could be epitaxially grown as thin film on a (100) SrTiO3 substrate in a tetragonally elongated rocksalt structure with lattice parameters of a = 3.905 Å and c = 5.32 Å, as a result of the Jahn-Teller effect [9] The existence of such an edge-connected T-CuO net is exciting as it will serve to support my ideas of cagey cluster formation and synthesis routes. A summary assessment of synthesis routes and crystal chemistry of infinite-layer compounds in contrast to the cubic cluster compound BaCuO2 suggests an explanation of cluster formation, leading to the proposal of a cuprate super-cage structure
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