Some old and some new facts in the perovskite based high temperature superconducting materials

The search for High Temperature Superconducting (HTSC) materials, has recently been strengthened by use of high pressure methods. This was due to the need to stabilise some noble metal cations - in particular, mercury - within the perovskite related structure of the superconducting cuprates. This is in fact due to the mercury oxides being metastable at the temperatures required for the reaction of the remaining components precursors, mainly carbonates, nitrates, oxides or peroxides of copper, the alkali-earth or the rare-earth metals. The use of high mechanical pressures allows mercury to be introduced in the structure and pure- and novel-compounds have, in this way, been obtained. in particular, the well known family: HgBa2Can-1CunO2n+2+delta (n = 1 - 7) has been prepared, showing the highest superconducting critical temperatures so far observed both at room temperature, T-c = 133.5 K and under pressure, T-c similar to 160 K at 35 GPa. By trying to replace mercury by other metals, and also under pressure, another family of non toxic compounds has been obtained. They correspond to the formula: (Cu-x C-1-x)Ba2Can-1CunO2n+2+delta (n = 3 - 6, 7?) and show quite high critical temperatures (T-c = 126 K at room pressure and 136 K under a pressure of 35 GPa). After a brief description of some interesting aspects of HTSC materials related to their having a crystal structure that derives from perovskite and that allow them to be structurally classified in terms of a general formula for a homologous series, some recent results concerning HTSC materials will be presented and discussed.