Paramagnetic resonance of Cu²⁺ defects in mixed n = 2 and n = 3 phase Bi_1.5Pb_0.5Sr₂Ca_n-1Cu_nO_2n+4 superconductor

Two EPR lines, one having an axially symmetric g tensor and the other an isotropic g tensor, are observed in mixed n = 2/n = 3 phase samples of the Bi_1.5Pb_0.5Sr₂Ca_n-1Cu_nO_2n+4 superconductor. No EPR spectra are, however, observed in phase pure (as determined by X-ray diffraction) n = 2 and n = 3 samples prepared under similar conditions. The EPR line intensities decrease as the temperature is lowered through T_c approximately following the decrease in resistance to zero of the samples. The spectra are attributed to point defects localized on copper-oxygen complexes at dislocations between n = 2 and n = 3 phase intergrowths, which have been identified in microcrystals of these samples by high resolution lattice imaging.