Double plateau T-c behaviour in the tetragonal LaBa1.5Ca0.5Cu3Oz

Slowly cooled samples of the tetragonal superconductor LaBa1.5Ca0.5Cu3Oz show a ''double plateau'' in plots of T-c against z, whereas quenched samples show a linear variation. No evidence for superstructure formation is observed by electron diffraction. Unit-cell parameters a and c vary linearly with x for slowly cooled samples, but non-linearly for quenched samples. For a given z, the difference in T-c of quenched and slow-cooled samples may indicate a net transfer of charge between the 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 oxygen vacancies.