Low-temperature electron paramagnetic resonance measurements were made of compounds with an O deficiency in the tetragonal phase. A nuclear hyperfine interaction of the Cu2+ ions was observed in both compounds. The origin of the Cu2+ electron paramagnetic resonance spectra was associated with localized divalent Cu ions in Cu(I) sites. A transformation of the Cu2+ local crystal-field symmetry, from orthorhombic to tetragonal, was detected which was related to the O distribution in the Cu(I) plane. The electron paramagnetic resonance spectra of pairs of weakly exchange-coupled divalent Cu ions were also studied. These Cu pairs were suggested to occur in the Cu(I) planes and to be magnetically isolated from the antiferromagnetic CuO2 planes.

V.Likodimos, N.Guskos, G.Palios, A.Koufoudakis, J.Typek, B.Bojanowski, M.Wabia: Physical Review B, 1995, 52[10], 7682-8