DEFECT CHEMISTRY OF BACUO2 .1. OXYGEN NONSTOICHIOMETRY, CATION MOLECULARITY AND X-RAY-DIFFRACTION DETERMINATIONS

''BaCuO2'' is the key intermediate in the synthesis of the Ba2YCu3O7-d elta superconductor. Its very complex crystal structure is able to acc ommodate a large change in oxygen content. Oxygen non-stoichiometry of ''BaCuO2'' materials with 1:1 and 88:90 (Ba:Cu) molecularity has been investigated by polythermal X-ray powder diffraction coupled with iso baric-isothermal gravimetry determinations under different temperature and oxygen partial pressure conditions [300 less than or equal to T l ess than or equal to 820 degrees C, 1 greater than or equal to P(O-2) greater than or equal to 3 . 10(-3) atm]. The 1:1 composition does not give well reproducible results, thus suggesting its polyphasic nature , at least in part of the investigated range. The results for the 88:9 0 congruent to 0.98 (Ba:Cu) composition are well reproducible and show that the material is single phase. Ba0.98CuO1.98+delta is oxygen over -stoichiometric in the whole investigated [T, P(O-2)] range, with a ma ximum value delta similar to 0.21. A Rietveld X-ray profile fitting is in agreement with previous single-crystal data. The trend of delta vs . P(O-2) is consistent with the presence of oxygen interstitial defect s on (possibly different) crystallographic sites.