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, eq uilibrated at 460 degrees C, have been prepared under reducing or oxid izing conditions from a well sintered master batch of nearly 100% comp actness. The investigation by the x-ray and neutron diffraction method s and by the resistivity, thermoelectric power, Hall effect, and magne tic susceptibility measurements have shown three distinct superconduct ing 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 equ al to y greater than or equal to 6.50) and are associated with the ort ho I and II structures which are characterized with infinite Cu-O chai ns. The third plateau corresponds to a macroscopically tetragonal phas e with T-c = 25 K (6.50 greater than or equal to y greater than or equ al to 6.40) and is possibly associated with formation of the Cu-O-Cu d imers. All three superconducting phases exhibit sharp magnetic transit ions with nearly complete diamagnetism at low temperatures and a metal liclike electric conductivity in the normal state. The hole concentrat ion as deduced from the calculated bond valence sums (BVS) smoothly de creases with decreasing y without apparent steps. For 6.89 greater tha n or equal to y greater than or equal to 6.36 the BVS values are in go od agreement with the number of itinerant carriers deduced from the Ha ll data. In the nonsuperconducting phase with y = 6.33, the sudden dec rease of the Hall number and simultaneous temperature activation of th e Hall constant and resistivity indicate a localization of carriers be low 300 K. Nevertheless, the Hall mobility remains compositionally ind ependent for all samples with 6.89 greater than or equal to y greater than or equal to 6.33.