A. Koufoudakis

XRD EPR studies of ceramics Pr0.5Re0.5Ba2Cu3O7−δ in the orthorhombic and tetragonal phase

EPR, XRD, and magnetic studies are presented for Pr0.5Re0.5Ba2Cu3O7−δ compounds (Re= La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic and tetragonal (large oxygen deficiency) phase. For the samples with Re=Dy, Ho, Y, Er, Tm, Yb, and Lu in the orthorhombic phase, the transition to the superconducting state has been observed in the temperatures rangeTc between 18 and 40 K. For the samples with Nd and Yb in the tetragonal phase, EPR spectra coming from trivalent rare earth ions have been recorded. In the nonoxygenated sample Pr0.5La0.5Ba2Cu3O7−δ the EPR spectrum arising from the non-Kramers trivalent praseodymium ion has been observed. A broad EPR line appearing in all our samples was attributed to superexchange interaction between copper ions over oxygen (O2−) bridges. Interestingly, for the Pr0.5Re0.5Ba0.5Cu3O7−δ (Re = Er and Lu) compounds in the tetragonal phase at liquid-nitrogen temperature, a nonresonant microwave absorption in low magnetic fields has been detected.

INVESTIGATION OF THE SAMARIUM ROLE IN THE PHYSICAL PROPERTIES OF Sm0.5Re0.5Ba2Cu3O7−δ COMPOUNDS IN THE ORTHORHOMBIC AND TETRAGONAL STRUCTURES

Perovskites of type Sm0.5Re0.5Ba2Cu3O7−δ (Re=Y, La, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) have been investigated by XRD, magnetic and EPR measurements in both orthorhombic and tetragonal phases. In the former case the usual superconducting phase transition in the range 90 to 95 K has been detected. In all the compounds an EPR spectrum of Cu2+ ions has been observed. In the tetragonal phase, the EPR spectra of trivalent rare earth ions have been recorded for some members of the series. The observation of a broad EPR signal, tentatively attributed to superexchange interaction over the oxygen bridges <O2), has also been recorded. Furthermore, the intensity of low-field microwave absorption has been found to depend strongly on the nature of the substituted rare earth ion at the sites of Y ions of the YBa2Cu3O7−δ type compounds.

On the Influence of Oxygen Deficiency upon EPR Spectra of TmBa2Cu3O7-δ

It is shown that for the samples with increased oxygen deficiency (non-superconducting tetragonal phase) one observes, apart from the signal due to Cu(+2), another clear signal which may be ascribed to the presence of Tm(+2) ions. For the superconducting sample (TC=91K, orthorhombic phase) this latter signal vanishes and one observes only the signal comming from Cu(+2) ions feeling a crystal field of rhombic symmetry.