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 resistivity, thermoelectric power, Hall effect and magnetic susceptibility measurements have shown three distinct superconducting plateaux depending on the decreasing oxygen content. The first two plateaux correspond to Tc 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 characterized with infinite Cu-O chains. The third plateau corresponds to macroscopically tetragonal phase with Tc=25 K (6.50>y>6.40) and is possibly associated with formation of the Cu-O-Cu dimers. For five selected samples (y=6.13-6.89), a complete structural determination was 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 resolution (Δd/d=0.007 in the minimum). The oxygen content y per formula unit of Y0.8Ca0.2Ba2Cu3Oy was estimated under the assumption of a linear relationship between the y value and the c lattice parameter. This correlation was deduced from the neutron diffraction results on selected samples completed with the value of the c-parameter of 1.1800 nm which was observed for a series of maximum reduction identified with stoichiometry y=6.0. (orig.)