Oxygen content and superconductivity in Y0.8Ca0.2Ba2Cu3Oy (y=6.03-6.89)

The Y0.8Ca0.2Ba2Cu3Oy samples with a large range of oxygen content, equilibrated at 460 degrees C, have been prepared under reducing or oxidizing conditions from a well sintered master batch of nearly 100% compactness. The investigation by the x-ray and neutron diffraction methods and by the resistivity, thermoelectric power, Hall effect, and magnetic susceptibility measurements have shown three distinct superconducting plateaus depending on the decreasing oxygen content. The first two plateaus correspond to T-c of about 80 K (6.89 greater than or equal to y greater than or equal to 6.75) and 50 K (6.75 greater than or equal to y greater than or equal to 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 a macroscopically tetragonal phase with T-c = 25 K (6.50 greater than or equal to y greater than or equal to 6.40) and is possibly associated with formation of the Cu-O-Cu dimers. All three superconducting phases exhibit sharp magnetic transitions with nearly complete diamagnetism at low temperatures and a metalliclike electric conductivity in the normal state. The hole concentration as deduced from the calculated bond valence sums (BVS) smoothly decreases with decreasing y without apparent steps. For 6.89 greater than or equal to y greater than or equal to 6.36 the BVS values are in good agreement with the number of itinerant carriers deduced from the Hall data. In the nonsuperconducting phase with y = 6.33, the sudden decrease of the Hall number and simultaneous temperature activation of the Hall constant and resistivity indicate a localization of carriers below 300 K. Nevertheless, the Hall mobility remains compositionally independent for all samples with 6.89 greater than or equal to y greater than or equal to 6.33.