Variations of stoichiometry and cell symmetry in YBa2Cu3O7−x with temperature and oxygen pressure
Variations of stoichiometry and cell symmetry in YBa2Cu3O7−x with temperature and oxygen pressure
复制标题
YBa2Cu3O7−x 的化学计量和晶胞对称性随温度和氧压的变化
作者：
P. Strobel;J. Capponi;C. Chaillout;M. Marezio;J. Tholence
铜的混合价态（Cu（II）-Cu（III））似乎在最近发现的钇钡铜氧化物YBa 2Cu 3 O 7-x的高临界温度超导电性中很重要;几位作者已经注意到合成过程对超导临界温度的影响。（参考文献1、2）。最近的结构测定3 - 5表明存在氧空位，将铜的配位数从理想化学计量的钙钛矿型化合物中的6改变为YBa 2Cu 3 O 7 −x中的4和5。在这里，我们报告了YBa 2Cu 3 O 7-x在不同温度和氧分压下进行的热重研究，沿着单晶X射线衍射数据显示晶体学转变作为温度的函数的证据。在通常的退火条件（800- 1，000 °C）下，即使在氧气气氛中，也不能达到最佳的铜价态，而只能在低于350 °C时达到。氧损失率随温度的变化可能与在空气中约600 °C时发生的斜方晶系到四方晶系的结构转变有关。
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 temperatureTc(refs 1, 2). Recent structure determinations3–5have 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−xcarried 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.