Variations of stoichiometry and cell symmetry in YBa2Cu3O7−x with temperature and oxygen pressure

• Published in: Nature (London) 1987-05, Vol.327 (6120), p.306-308
• Main Authors: Strobel, P., Capponi, J. J., Chaillout, C., Marezio, M., Tholence, J. L.
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Physics; Science; Science (multidisciplinary); Structure of solids and liquids; crystallography
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/327306a0

The mixed valence of copper (Cu(II)–Cu(III)) seems to be important in the recently discovered high-critical-temperature superconductivity of the yttrium barium copper oxide, YBa 2 Cu 3 O 7− x ; several authors have noted the influence of synthesis procedures on the superconducting critical temperature T c (refs 1, 2). Recent structure determinations 3–5 have shown the presence of oxygen vacancies, changing the copper coordination number from 6 in an ideal stoichiometric perovskite-type compound to 4 and 5 in YBa 2 Cu 3 O 7− x . Here we report a thermogravimetric study of YBa 2 Cu 3 O 7− x carried out at various temperatures and oxygen partial pressures, along with single-crystal X-ray diffraction data showing evidence of a crystallographic transformation as a function of temperature. The optimum copper valence is not reached under usual annealing conditions (800–1,000 °C), even in an oxygen atmosphere, but only below ∼350 °C. A change in the rate of oxygen loss with temperature may be related to an orthorhombic to tetragonal structural transformation occurring at ∼600 °C in air.