A Composite Ionic-Electronic Conductor in the (Ca, Sr, Ba)-Bi Oxide System
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A Composite Ionic-Electronic Conductor in the (Ca,Sr,Ba)-Bi Oxide System James K. Meen, Oya A. Gökçen, I-C. Lin, Karoline Müller, and Binh Nguyen Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, U.S.A. ABSTRACT The rhombohedral alkaline earth-bismuth oxide phase, an oxygen ion conductor, does not coexist stably with electronic conductors in any of the three binary systems, Ca-Bi-O, Sr-Bi-O, Ba-Bi-O. A thermodynamically stable composite of a rhombohedral phase that contains Ba and Sr or Ca or both with the electronic conductor BaBiO3 may be synthesized. The rhombohedral phase appears to have complete mutual miscibility of the alkaline earth elements. The compositions of rhombohedral phase that coexist with BaBiO3 in the Sr-Ba-Bi ternary system and the CaSr-Ba-Bi quaternary systems are described. The value of ionic conductivity of the rhombohedral phase (at a constant Bi: [Ca+Sr+Ba]) is not dependent on the relative amounts of Ca, Sr, and Ba. The temperature at which the rhombohedral phase undergoes a polymorphic transformation from a low-temperature (β 2) form that is a weak ion conductor to a high-temperature (β 1) form that is a much better oxygen ion conductor. The temperatures of the polymorphic transformation and of the upper stability limit of the rhombohedral phase both depend strongly on Ca: Sr: Ba. The β 1 form develops in the Ba-Bi system at the lowest temperatures and at the highest ones in the CaBi system. On the other hand, the Ca-Bi phase has greater thermal stability than its Ba analogues. The temperature range over which a useful composite conductor can operate is, therefore, strongly dependent upon the bulk composition of the system.
INTRODUCTION The rhombohedral AE-Bi oxide (where AE is an alkaline earth element, Ca, Sr, or Ba) phase is a well-known oxygen ion conductor. The Ba-Bi-O system also contains BaBiO3, an electronic conductor. Rhombohedral Ba-Bi oxide and BaBiO3 do not coexist stably [1]. Addition of Ca or Sr or both to the system results in production of a two-phase field in which rhombohedral AE-Bi oxide and BaBiO3 coexist over a temperature range. A composite ionic-electronic conductor may be thus created. In this composite, the composition of the perovskite remains essentially constant but the rhombohedral phase has a wide range of alkaline earth contents. The two-phase field is terminated by three-phase fields that limit the range of compositions of rhombohedral phase that may be incorporated into a composite ionic-electronic conductor with the perovskite. The properties of the composite ionic-electronic conductor are strongly influenced by the composition of the rhombohedral phase. The composition of the rhombohedral phase determines both the temperature of the polymorphic transformation and the temperature of decomposition of the biphasic assemblage. The low-temperature $2 polymorph transforms to a high-temperature $1 form through displacive transformation that involves no change in space group (R-3m) [2,3]. A sudden and ma
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