PRECISE DETERMINATION OF THE HYPERFINE PARAMETERS OF EUROPIUM IN MULTIFLUORITE PEROVSKITES BY EU-151 MOSSBAUER-SPECTROSCOPY

The hyperfine interactions at the europium lattice sites in samples of the homologous (Fe,Cu)Sr2(Eu,Ce)nCu2O4+2n+z (n = 2,3) series were stu died by Eu-151 Mossbauer spectroscopy. The work was motivated by the s earch for new superconducting phases. This homologous series is based on the YBa2Cu3O7-delta (1:2:3) structure. The samples used in the Moss bauer measurements consisted of crystallites with random orientation a nd grain oriented crystallites. The texture of oriented samples was an alyzed by x-ray diffraction. The complete quadrupole Hamiltonian of th e 21.5-keV gamma-transition of Eu-151 was successfully applied in the analyses of all the Mossbauer spectra. In samples having n less-than-o r-equal-to 2 the europium atoms occupy a single lattice site, whereas the spectra of the n = 3 samples exhibit hyperfine interactions of the two different europium sites. Analyzing the hyperfine parameters of t he latter samples was made possible by simultaneous fitting of three s pectra, corresponding to three different crystal orientations of the s ame specimen. This fitting scheme also enables more precise determinat ion of the hyperfine parameters in the n = 2 samples. In these samples an electric field gradient (EFG), with a large negative-valued main c omponent (V(zz)) parallel with the crystal c axis, was found. In the n = 3 samples, the two EFG's of the europium sites were found to have V (zz) components of opposite signs. The negative V(zz) value was attrib uted to the rare-earth site adjacent to the CuO layer. This site, foun d in all samples of the series, corresponds to the rare-earth site of the 1:2:3 system.