INFLUENCE OF THERMAL-TREATMENT ON Y1-XCAXBA2CU3-XCOXO7 - A NEUTRON-DIFFRACTION STUDY

Two series of Y1-xCaxBa2Cu3-xCoxO7 oxides have been synthesized by two different thermal treatments for x=0.12, 0.24, 0.30 and 0.36. The str ucture of these oxides has been determined by neutron powder diffracti on. The standard treatment under oxygen leads to a transition from ort horhombic to tetragonal for x greater-than-or-equal-to 0.09. These sam ples were then reduced under argon and reoxidized under oxygen. This t hermal treatment allows one to obtain the stabilization of the orthorh ombic domain until x=0.30. This study shows the ability of cobalt to m igrate into the Cu(2) sites. However, this ability is much smaller tha n in the case of iron. These results confirm the important role of the Cu(2)-pyramidal layers in the superconducting properties of ''123'' o xides.