STABILITY OF BA, SR AND CA CUPRATE PHASES AS A FUNCTION OF OXYGEN-PRESSURE

Alkaline earth-copper oxide binary mixtures were reacted in elevated o xygen pressure up to 200 bar in the temperature range 880-950-degrees- C. The phases which were formed have been identifed by X-ray powder di ffraction analysis. The compounds which become stabilized at higher P( O2) are Ba2Cu3O5+delta, Sr2CuO3+delta, (Sr14Cu24)1-x)O41 and Ca4Cu5O10 . These compounds have formal copper valence (FCV) in the range of 2.4 to 2.8 and are therefore favored at higher P(O2) over the ABO2 and A2 BO3 compounds obtained at 1 bar which have FCV = 2. The transition pre ssures to the more oxidized cuprates are approximately 10 bar for Ca2C uO3/Ca4Cu5O10, 17.5 bar for BaCuO2/Ba2Cu3O5+delta and 40 bar for SrCuO 2/(Sr14Cu24)1-xO41 at 880-950-degrees-C.