Characterization of large YBa 2Cu 3O 7-x single crystals by neutron diffraction

Two large YBa ₂Cu ₃O _{7- x} crystals with dimensions 3X2X2 mm ³, grown by a CuO-BaO self-flux method, were characterized by neutron diffraction. One crystal, taken directly from the flux, consists of a main oxygen deficient tetragonal part in the crystal center surrounded by a spectrum of orthorhombic phases with different oxygen concentrations between 6.45 and 6.75 and an averaged oxygen content of about 6.55(5). The mosaic angle of the tetragonal and orthorhombic phases is smaller than 0.3°. The orientation of the twinned orthorhombic axes are given by a (110) twin law. A second crystal was reloaded with oxygen. The result is an orthorhombic single crystal with a homogeneous oxygen concentration of 6.85(5) and a T _c, measured in a self-balanced magnetoviscosimeter, of 91.5(5) K. In this crystal there is no (110) twin law which is explained by a coherent superposition of mosaic microtwins.