Solid State Ionics 36 (1989) 85-88 North-Holland, Amsterdam MECHANISM OF OXYGEN INCORPORATION IN THE YBa2Cu306.9 SUPERCONDUCTOR F. CROCE, B. SCROSATI Dipartimento di Chimica, Universit~di Roma, "La Sapienza", Rome, Italy and C. ALVANI, S. CASADIO ENEA/TIB/CHI, CRE-Casaccia, Rome Italy Received 24 March 1989;acceptedfor publication 20 April 1989 It is well known that the oxygencontent exherts a critical role in determining the electrical characteristics of the perovskite cuprate superconductors. However,the formal oxidation state of copper in the compoundsas well as the mechanism of oxygen incorporation in their structure, are still a matter of discussion. In this work, we propose two possible disorder equilibria in the 400°C to 525°C temperature range and attempt to substantiate them by experimental evidence. 1. Introduction It has now been certain [ 1,2 ] that oxygen is cru- cial in controlling the electrical behaviour of the re- cently discovered perovskite cuprate superconduc- tors [3-5]. These compounds appear to contain copper in a formal oxidation state greater than two, which is controlled by the oxygen content. Thus, the superconductivity characteristics are greatly affected by the annealing conditions of the samples, being particularly sensitive to oxygen partial pressure and to temperature. The Ba2YCu307_x type materials undergo a te- tragonal-orthorhombic phase transition at a temper- ature which greatly depends upon oxygen partial pressure, varying from 530°C under 1% of oxygen to 700 ° C under 100% of oxygen [ 1 ]. In air (21% ox- ygen) the transition temperature is of the order of 650°C. At low oxygen partial pressure (i.e. <0.01%), a change in the electric properties of the materials (which pass from a metallic to a semiconducting be- haviour) has also been evidenced [ 1 ]. Therefore, it has seemed to us of importance to further investigate the role of oxygen in the cuprate superconductors and in this work we propose a mechanism for the process of oxygen incorporation in YBa2fu306.9 in the temperature range of 400 to 525 °C. 2. Experimental The YBa2Cu306.9 compound was prepared from mixed powders of CuO, BaO and Y203, heated at 900 ° C for a few hours. The material was then pressed and sintered in air at 950°C for about seven hours with a slow cooling to room temperature. The geo- metrical density was 5.08 g/cm 3, corresponding to the 79.4% of the theoretical values 6,4 g/cm 3. X-ray analysis indicates that our specimens are single phase, comparable with the best ceramic samples reported elsewhere. The samples, cut into bars with a rectangular base having the dimensions of 30 × 4 X 2 ram, were kept in a controlled environment cell situated in an oven, in order to perform the desired resistivity test under various atmospheres and temperature conditions. A four-probe technique was used for the resistivity test. Silver wire terminals were attached to the sample by silver paste. The measurements were carried out both under dc and ac polarizations using a Solartron Fre- quency Response Analyser, model 1250 coupled with 0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )