Transport Properties And Crystal Chemistry Of Ba-Sr-Bi Oxides

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Transport Properties And Crystal Chemistry Of Ba-Sr-Bi Oxides Oya A. Gökçen, James K. Meen, Allan J. Jacobson and Don Elthon Department of Chemistry, Texas Center for Superconductivity, and Materials Research Science and Engineering Center, University of Houston, Houston, Texas 77204, U.S.A. ABSTRACT Rhombohedral (R) phases of the binary systems AE-Bi-O (AE = Ca, Sr, Ba) have low temperature ($2) and high temperature ($1) polymorphs and the $1 polymorph is a good oxygen ion conductor. In a previous study, we showed that in the ternary system Ba-Sr-Bi-O, conductivities of polycrystalline R samples are unaffected by Ba:Sr, whereas temperature ranges of the $2 ↔ $1 polymorphic transition are. Recently, we proved that the conductivity of both polycrystalline and single-crystal R samples is sensitive to spatial direction. For single crystals, the conductivity is highest perpendicular to the c-axis. Along the conduction plane, the conductivities are significantly higher than those of polycrystalline R samples; perpendicular to that plane they are significantly lower. The BaBiO3 perovskite (P) phase, predominantly an electronic conductor, has conductivities of 10 to 102 S/cm at 350-800°C in air. Conductivity measurements of the R-P assemblages with different R:P showed that percolation threshold is around 35 volume % P.

INTRODUCTION The β 1 high temperature polymorph of the R solid solution of the AE-Bi oxide binary systems (AE = Ca, Sr, Ba) and of the Ba-Sr-Bi oxide ternary system is a very good oxygen ionconductor with conductivities much higher than those of the stabilized zirconias at around 700°C. The conductivities of the R phase of the AE-Bi oxide binary systems [1-3] and those of the R phase of the Ba-Sr-Bi oxide ternary system [4-6] have been investigated by different authors. A recent work [7] on phase equilibria of the R solid solution in the Ba-Sr-Ca-Bi oxide quaternary system reveals the polymorphic transition temperatures with respect to alkaline-earth content of this phase. In each case, the polymorphic transition from the β2 polymorph to the β 1 polymorph was marked with sudden increase in conductivity. Mercurio et al. [8] studied the crystal structure of R specimens in AE-Bi oxide binary systems and suggested a plausible oxygen ion conductivity mechanism. The structure is characterized as a repetition of three fluoritelike sheets separated by an intersheet space free of anions for the β2 polymorph with the oxygen ions located within the fluorite-like sheet side of the structure. The movement of the oxygen ions from the fluorite-like sheet to the intersheet space causes the β 2 ↔ β1 transition. This transition is accompanied by an increase in the thickness of the intersheet space as well as an increase in the conductivity because the oxygen ions now occupy this corridor and can freely move along it. Thus, the main oxygen ion conductivity mechanism that is responsible for the high ionic conductivities of the β 1 phase is along the conduction planes perpendicular to the c-axis. In the Ba-Sr-Bi oxide ternary,

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