DOUBLE PLATEAU T-C BEHAVIOR IN THE TETRAGONAL LABA1.5CA0.5CU3OZ

Slowly cooled samples of the tetragonal superconductor LaBa1.5Ca0.5Cu3 Oz show a ''double plateau'' in plots of T-c against z, whereas quench ed samples show a linear variation. No evidence for superstructure for mation is observed by electron diffraction. Unit-cell parameters a and c vary linearly with x for slowly cooled samples, but non-linearly fo r quenched samples. For a given z, the difference in T-c of quenched a nd slow-cooled samples may indicate a net transfer of charge between t he Cu(2)-oxygen planes and the Cu(1)-oxygen chains which is dependent on the thermal history of the samples rather than on ordering of oxyge n vacancies.