STRUCTURAL DEFORMATION IN OXIDIZED AND REDUCED Y)(3)O-6-LESS-THAN-Y-LESS-THAN-OR-EQUAL-TO-0.05)(X COMPOUNDS (0)

We have investigated in detail the structural properties of the system YBa2 (Cu1-yCoy)(3)O-6+x With different values of x and y. X-ray diffr action shows that the Co doping induces an orthorhombic-to-tetragonal transition at y approximate to 0.025. Electron diffraction and electro n microscopy indicate that for y<0.025, displacement fields oriented a long the [110] directions exist throughout the crystallite. For y>0.02 5 electron diffraction shows the presence of intense streaks of diffus e scattering located at the tails of the Bragg reflections in the (001 ) plane of the reciprocal space, and oriented along the [110] directio ns. For the same oxidation treatment the size of the diffuse streaks i ncreases with increasing dopant concentration. They disappear when the sample undergoes a reduction. Two-beam images show a contrasted cross -hatched pattern whose hatchings are also oriented along the [110] dir ections. These features are displacive in origin and correspond to the superposition of transverse displacement fields whose propagation vec tors are oriented along the [110] directions. X-ray diffraction experi ments with a conventional source as well as with synchrotron radiation at LURE were carried out on a well oxidized single crystal (y=0.04). The diffuse scattering is of the same type as that seen with electron diffraction. These X-ray diffraction experiments confirms that the dif fuse scattering is displacive in origin and shows that the displacemen ts concern all atoms. The displacements are linked to the deformations created by the specific arrangement of the oxygen atoms forming the - Cu1-O4 microchains. One way to relax the internal constrains due to th e microchains is to form microtwins. The lattice is distorted at each twin wall and this distortion induces transverse zig-zag displacements of all atoms, perpendicular to the twin walls.