Oxide superconductors, metastability

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

The superconducting 1-2-3 phases are known to be chemically sensitive, and in fact are bought to be metastable compounds imder all conditions of temperature and oxygen partial pressure (1). Thus, it is imperative that any material in intimate contact with 1-2-3 phases does not react with the component oxides to form more stable compounds, since such a reaction will destroy the superconducting material. This is especially important for thin fflm applications, since the amount of superconductor is small compared to that of the substrate, and the diffusion path for potential solid state reactions, i.e., the film thickness, is very short. In this paper we will concentrate on searching for materials that should be most stable in contact with the 1-2-3 superconductors, since they appear to be more sensitive to chemical disruption than the more recently discovered Bi- and Tl-based phases. The principles and much of the data presented here can be applied to any oxide superconductor, whether it is presently known or yet to be discovered. 

There are two possible exceptions to the rule that high Tc superconductors are metastable. Both YBa2Cu4Og and Y2Ba4Cu7015 appear to be stable under the conditions where they form, and they are superconductors without further oxidation. The remaining question to be answered is whether or not they are thermodynamically stable at room temperature and below. This is a difficult question to answer. Calorimetry can compare the heats of formation of these compounds with other compounds known in the Y/Ba/Cu/O system. However, it is always possible that some of the most stable phases in the Y/Ba/Cu/O system have not yet been prepared because they are kinetically inaccessible. 

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