Y-Ba-Cu-O system

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

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. 

Figure 3. Compositions of Y-Ba-Cu-O system and summary of results for samples prepared at (a) 875 °C and (b) 1050 °C. Continued on next page.

Details of the nomenclature and the importance of the charge reservoir block, as weU as the doping level, are explained below for the Y-Ba-Cu-O system. 

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