Anion substitutions and insertions in copper oxide superconductors

After the discovery of high-Tc superconductivity in multinary cuprates, a large number of different superconductors have been synthesized by varying the chemical composition and structural features of the insulating layers which connect the superconducting Cu-O planes. Although experimental studies initially focused on aliovalent cationic substitutions in these layers, it was subsequently discovered that oxyanion groups - (CO3)2-, (NO3)-, (BO3)3-, (SO4)2-, (PO4)3- - can also be located on some cationic sites to provide a further means of controlling the electronic properties of the superconducting regions. Even more recent developments have allowed the study of oxide-halide superconductors, synthesized by 'soft chemistry' or high pressure techniques aimed at overcoming the thermodynamic instability of most of these materials. Superconductivity in these new families of oxide halides is of particular interest since critical temperatures appear to be at least as high as oxide analogues, and it provides a valuable guide to the importance of structural features, e.g. the apical anion, for superconducting properties. (author)