3-D oxygen vacancy ordered superstructures in YBa2Cu3O7t-δ prepared by the constant stoichiometry cooling method

Abstract In very slowly cooled (25°C/h) specimens prepared following the constant stoichiometry method, the 3D structure of the orthorhombic II (2a o ) phase in YBa 2 Cu 3 O 7−δ was determined using electron diffraction and electron microscopy. The unit cell is 2a o x b o x c o ; successive 2D configurations in the CuO 1−δ layers are aligned along the c-direction. Evidence is presented for the occurence of more complicated 2D superstructures: 3a o , 4a o and 5a o . It is suggested that the a o 2√2 × a o 2√2 structure is a deformation modulated phase, rather than a vacancy ordered one. The O II structure is the most and probably the only vacancy ordered stable phase apart from the O I and the tetragonal phases. The more complicated na o phases are probably metastable. The disordering with increasing temperature of the O II structure occurs in different stages.