Superconductivity at High Temperatures Without Copper: Ba1-xKxBiO3

Abstract

Before the pioneering work of Bednorz and Müller in finding superconductivity near 30 K in lanthanum-barium-copper oxide,. oxide superconductors were well known, but perhaps not fully appreciated. Most anomalous among those superconductors was the perovskite structure material BaPb0.75Bi0.25O3, with a superconducting transition temperature (Tc) of 12 K2 and a surprisingly low density of states at the Fermi level. The increases in Tc for copper-oxide-based materials continue to generate worldwide excitement, but from both a chemical and theoretical point of view, high Tc superconductivity observed in a noncopper containing material is also of considerable interest.Recently we found that the simple cubic perovskite compound Ba0.6K0.4BiO3 displays a superconducting transition temperature near 30K—a Tc considerably higher than that of conventional superconductors and surpassed only by copper containing compounds. This material is in stark contrast to the now well-known copper oxides for two reasons: (1) superconductivity occurs within the framework of a three dimensionally connected bismuth-oxygen array (and not a 2-d array as in the Cu-O based compounds) and: (2) there are no magnetic fluctuations present in the chemical system, either in the superconductor itself or in the nonsuperconducting end member compound, eliminating the possibility that the high Tc might be caused by magnetic interactions. The parent compound BaBiO3 is, however, of considerable interest due to the presence of a structurally frozen charge disproportionation of the bismuth atoms, considered by many to be the electronic equivalent of the antiferromagnetism observed in the nonsuperconducting cuprate host compounds.The ideal undistorted perovskite ABO3 structure consists of a regular array of equally dimensioned BO6 octahedra sharing all corner oxygens with neighboring equivalent octahedra, with 180° B-O-B angles.