Variations of stoichiometry and cell symmetry in YBa₂Cu₃O_7−x with temperature and oxygen pressure

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, YBa₂Cu₃O_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 YBa₂Cu₃O_7−x. Here we report a thermogravimetric study of YBa₂Cu₃O_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.