STRUCTURE PROPERTIES AND OXYGEN-CONTENT IN Y0.6CA0.2BA2CU3OY (Y=6.03-6.89) BY NEUTRON-DIFFRACTION

Investigations by the neutron and X-ray diffraction methods and by res istivity, thermoelectric power, Hall effect and magnetic susceptibilit y measurements have shown three distinct superconducting plateaux depe nding on the decreasing oxygen content. The first two plateaux corresp ond to T-c of about 80 K (6.89 > y > 6.75) and 50 K (6.75 > y > 6.50) and are associated with the Ortho I and II structures which are charac terized with infinite Cu-O chains. The third plateau corresponds to ma croscopically tetragonal phase with T-c = 25 K (6.50 > y > 6.40) and i s possibly associated with formation of the Cu-O Cu dimers. For five s elected samples (y = 6.13-5.89), a complete structural determination w as achieved by neutron diffraction. The experiments were performed at room temperature on the diffractometer KSN-2 in Rez near Prague using the neutron wavelength of 0.1365 nm and the arrangement of the best re solution (Delta d/d = 0.007 in the minimum). The oxygen content y per formula unit of Y0.8Ca0.2Ba2Cu3Oy was estimated under the assumption o f a linear relationship between the y value and the c lattice paramete r. This correlation was deduced from the neutron diffraction results o n selected samples completed with the value of the c-parameter of 1.18 00 nm which was observed for a series of maximum reduction identified with stoichiometry y = 6.0.