Structure and superconductivity of BaBiO 3 doped with alkali ions
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I. INTRODUCTION
BaPbi ^Bi^Os can be considered as the precursor material of today's high temperature oxide superconductors. It is a mixed crystal of BaPbO3 and BaBiO3, with the mixing of Bi and Pb occurring at the B-sites of the perovskite ABO3 structure. For x < 0.3, metallic and superconducting properties have been observed by Sleight et al.1 However, one of the end members, namely BaBiO3, is a semiconductor. If the oxidation state of Bi in the latter compound would be 4+, one would expect the compound to have metallic conductivity due to the lone valence electron associated with each Bi ion. A neutral Bi atom has five (6s2p3) valence electrons and the valence of Bi in its compounds is either 3 or 5. Neutron powder diffraction2 indicates that in BaBiO3, Bi ions exist in a mixed valence state which can be represented as Ba2Bi3+Bi5+O6. Since the ionic radius of Bi3+ is considerably larger than that of Bi5+, we have a situation in which the oxygen octahedra surrounding the Bi ions are alternately expanded and compressed (the so-called breathing displacement). It has been shown that this type of lattice distortion generates an energy gap which explains the semiconducting characteristic of BaBiO3.3 In 1988 it was discovered that this compound could become superconducting near 30 K by its doping with alkali ions, K or Rb.3'4 This perovskite related system constitutes a unique example of copperfree oxide superconductor. It also provides distinct features which are not commonly found in the cuprate superconductors. In the nonmetallic phases of the cuprate, La2CuO4 for instance, antiferromagnetic order has been found to occur. This has led to the speculation that this ordering may extend into the superconducting region as well and mediate the Cooper pairing. BaBiO3 is not antiferromagnetic but diamagnetic. Thus, the magnetic pairing mechanism is not likely to
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http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 4, Apr 1990
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be applicable to this system. The Bi compound has a three-dimensional network which differs considerably from the layered structure of the cuprate superconductors. Thus, when building theoretical models to explain the high-temperature superconductivity, one must take into consideration these differences between the two systems. Less effort has been directed to the development of alkali doped bismuth-base superconductors for various reasons. First, the incorporation of alkali ions into the BaBiO3 lattice is difficult to achieve because of the volatility of the former. Cava et al.4 calcined a mixture of appropriate oxides in a sealed silver tube to prevent the loss of potassium. The powder thus obtained had a magnetically determined onset temperature of 29.8 K but was not suitable for resistivity measurements. Hinks et al.5 calcined the oxide mixture in a N2 atmosphere at 700 °C followed by annealing in O2 at lower temperatures. Samples suitable for resistivity measurements were thus synthesized. Recently, we reported that
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