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

TL;DR: In this article, a thermogravimetric study of YBa2Cu3O7−x was 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.

Abstract: 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, YBa2Cu3O7−x; several authors have noted the influence of synthesis procedures on the superconducting critical temperature Tc (refs 1, 2). Recent structure determinations3–5 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 YBa2Cu3O7−x. Here we report a thermogravimetric study of YBa2Cu3O7−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.