Powdered samples of the Pr0.5RE0.5Ba2Cu3O6 + x compounds, where RE Yb, Ho, Tm, Er, and Y, were investigated by the EPR technique at low temperatures in the range 2.8-20 K. In the EPR spectra of these non-superconducting materials in tetragonal phase, resonance lines arising from the exchange interaction of coupled pairs of divalent copper ions and Cu2+ in the tetragonal local crystal field symmetry were recorded. Apart from the usually observed ΔMs = 1 and ΔMs = 2 transitions for the CuCu system, rarely reported singlet-triplet transitions were detected as well. The exchange parameter J has roughly the same value of about 0.12 cm-1 for all investigated samples and the zero-field splitting parameter D is of the order of 0.013 cm-1. It is suggested that formation of Cu(1)OCu(2) copper coupled pairs is responsible for the observed EPR spectrum. © 1995.

Low-temperature EPR measurements are presented for the PrBa2Cu3O6+x and Pr0.5R0.5Ba2Cu3O6+x (R=Y,Er) compounds with oxygen deficiency (tetragonal phase). The nuclear hyperfine interaction of the Cu2+ ions has been observed in both compounds. The origin of the Cu2+ EPR spectra has been associated with localized divalent copper ions in the Cu(1) sites. A time-induced transformation of the Cu2+ local crystal-field symmetry from orthorhombic to tetragonal, related to the oxygen distribution in the Cu(1) plane, has been detected. The EPR spectra of pairs of weakly exchange-coupled divalent copper ions were also observed. These copper pairs are suggested to occur in the Cu(1) planes and to be magnetically isolated from the antiferromagnetic CuO2 planes. © 1995 The American Physical Society.

The polymorphic modifications α-, β-, and γ-Fe2WO6 of the iron tungstate system were studied by means of magnetic susceptibility and EPR measurements at low temperatures. Both methods revealed a significant paramagnetic contribution, probably resulting from local distortions of the antiferromagnetic bulk structure induced by a disturbed cation ordering or the presence of Fe2+ ions. The magnetic susceptibility revealed a peak at ∼260 K for all samples which can be related with an AF phase transition. The EPR spectra comprised the contribution of various isolated paramagnetic iron centers, one arising from high-spin Fe3+ ions in rhombic crystal field symmetry with E/D ≈ 1/3 and D ≈ 0.22 cm-1, an anisotropic EPR signal consistent with an S = 3/2 ground state with large zero-field splitting, and a dominant component in the g ≈ 2 region presumably arising from an S = 1/2; spin state. The latter spectra were tentatively attributed to the formation of multi-iron clusters, one of them invoking the presence of Fe2+ ions as well. For the βFe2WO6 phase an additional EPR spectrum was observed, which probably results from high-spin Fe3+ ions in a weak crystal field. © 1995 Academic Press. All rights reserved.

XRD, magnetic susceptibility, and EPR studies are reported for the BaCuO2+x compound in both the oxygenated and nonoxygenated phases. The XRD analysis has shown essential differences in the disordered part between the oxygenated and the nonoxygenated samples. The magnetic susceptibility measurements have shown strong ferromagnetic intracluster interactions among copper ions in the ordered part of the compound. At low temperature, antiferromagnetic intercluster interactions mediated by the disordered part predominate. EPR measurements in both phases have revealed a spectrum of Cu2+ ions, in orthorhombic local symmetry, which is attributed to copper ions in the disordered part. An intense exchange-narrowed EPR line has also been observed in both phases, which is ascribed to the copper clusters in the ordered part. The temperature dependence of the intensity and linewidth of this line is consistent with the bulk magnetic behaviour. At low temperature, the divergent behaviour of the EPR parameters indicates that antiferromagnetic ordering may occur. Additionally, it was found that the EPR spectra are essentially influenced by thermal treatment and the ordering of the oxygen atoms in the disordered part of the compound. © 1995 Academic Press Limited.