It was recalled that so-called BaCuO2 was the key intermediate for the synthesis of the present superconductor. Its very complex crystal structure could accommodate a large change in O content. The O non-stoichiometry of BaCuO2 materials with a 1:1 or 88:90 Ba/Cu ratio was investigated by means of polythermal X-ray powder diffraction, coupled with isobaric-isothermal gravimetric determinations at temperatures of between 330 and 820C and at O partial pressures of between 0.003 and 1atm. It was found that the 1:1 composition did not give reproducible results; thus suggesting that it was polyphase within at least part of the investigated ranges. The results for the 88:90 composition were reproducible, and showed that the material was monophase. It was noted that Ba0.98CuO1.98 was O hyper-stoichiometric within the whole of the investigated temperature and pressure ranges. The variation of the deviation from stoichiometry, as a function of the O partial pressure, was consistent with the presence of O interstitial defects; perhaps on different crystallographic sites.

G.Spinolo, U.Anselmi-Tamburini, M.Arimondi, P.Ghigna, G.Flor: Zeitschrift für Naturforschung, 1995, 50a[11], 1050-8