DISCOVERY OF A 2ND FAMILY OF BISMUTH-OXIDE-BASED SUPERCONDUCTORS

The superconducting oxide BaPb1-xBixO3, discovered in 1975 (ref. 1), i s an exotic system having an unusually high transition temperature (T- c) of similar to 12 K, despite a relatively low density of states at t he Fermi level. The subsequent prediction(2) that doping the electroni cally inactive barium donor sites, instead of the bismuth sites, might induce superconductivity with a higher T-c led to the discovery(3,4) in 1988 of superconductivity in the Ba1-xKxBiO3 system (T-c similar to 30 K for x = 0.4). But it remains an open question why many of the su perconducting properties of these materials are similar to those of th e well-known copper oxide superconductors(5), despite their pronounced structural differences: the former have a three-dimensional bismuth-o xygen framework, whereas the structures of the latter are predominantl y two-dimensional, consisting of copper-oxygen planes. Understanding o f the copper oxide superconductors has gained immensely from the study of many different superconducting systems, and so it might be expecte d that the identification of bismuth oxide superconductors beyond the substituted BaBiO3 compounds will prove to be similarly fruitful. Here we report the synthesis of a second family of superconducting bismuth oxides, based on SrBiO3. We show that partial substitution of potassi um or rubidium for strontium induces superconductivity with T-c values of similar to 12 K for Sr1-xKxBiO3 (x=0.45-0.6) and similar to 13 K f or Sr1-xRbxBiO3 (x = 0.5).