Existence of the homologous series of Y n Ba m Cu m + n O y (m = 2, 3, 5; n = 1, 2) oxides with the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ

Abstract

The phase composition of YxBa1−xCuOy (x = 0.29−0.40) samples annealed in air (at 930–990°C) and in an oxygen atmosphere (450–800°C, P(O2) = 101 kPa) was studied by X-ray powder diffraction, chemical analysis, electron diffraction, and elemental analysis in a transmission electron microscope. A considerable cation nonstoichiometry was discovered in particles having the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ. The variation range of particle compositions comprises matrix oxides of the BamCum + nOy series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the YnBamCum + nOy series. Tetragonal oxides Y2Ba3Cu5Oy (235), Y3Ba5Cu8Oy (358), YBa2Cu3Oy (123), and Y2Ba5Cu7Oy (257) are formed at the primary synthesis step in air and are preserved in an orthorhombic structure during short-term (1 h) oxygen annealing. Most particles of the 3: 5 and 5: 8 oxides are undersaturated with yttrium relative to the stoichiometry of the YnBamCum + nOy series, those of the 2: 3 oxide correspond to this stoichiometry, and those of the 5: 7 oxide are supersaturated with yttrium over the stoichiometry. A trend is observed for the fractions of these oxides to change during long-term (5–51 h) annealing in an oxygen atmosphere at 450°C and to the alternation of the dominant role of one of the four phases with the superconducting transition temperature Tc = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in Tc. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.