Browsing by Author "Katila, T."

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  • 151Eu Mössbauer spectroscopy and x-ray-diffraction studies on the Pb2Ba2EuCu3O8+[delta] system
    (1994) Lindén, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Karppinen, Maarit; Niinistö, L.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Simultaneous replacement of Sr by Ba and Y by Eu in the Pb-2213 system was found to yield single-phase Pb2Ba2EuCu3O8+δ samples, suitable for Eu151 Mössbauer measurements. The samples were synthesized by a solid-state reaction of metal oxides and carbonates under an inert atmosphere. An oxygen-rich sample corresponding to δ=1.79 was obtained by annealing the as-synthesized material (δ=0.16) in oxygen. The oxygen annealing increased the size of the lattice constants and led to an orthorhombic-to-tetragonal phase transition of the structure. The samples were also oriented in an 11.7-T magnetic field. The x-ray-diffraction spectra showed that the c axes tended to orient perpendicular to the applied field. The measured Mössbauer spectra exhibited an electric quadrupole interaction typical of the Eu site in high-Tc cuprates. The average orientation angles obtained from fittings of the Mössbauer spectra were in accordance with the results from the x-ray-diffraction measurements.
  • 151Eu Mössbauer spectroscopy and x-ray-diffraction studies on the Pb2Ba2EuCu3O8+δ system
    (1994) Linden, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Karppinen, M.; Niinistö, L.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Simultaneous replacement of Sr by Ba and Y by Eu in the Pb-2213 system was found to yield single-phase Pb2Ba2EuCu3O8+δ samples, suitable for 151Eu Mössbauer measurements. The samples were synthesized by a solid-state reaction of metal oxides and carbonates under an inert atmosphere. An oxygen-rich sample corresponding to δ=1.79 was obtained by annealing the as-synthesized material (δ=0.16) in oxygen. The oxygen annealing increased the size of the lattice constants and led to an orthorhombic-to-tetragonal phase transition of the structure. The samples were also oriented in an 11.7-T magnetic field. The x-ray-diffraction spectra showed that the c axes tended to orient perpendicular to the applied field. The measured Mössbauer spectra exhibited an electric quadrupole interaction typical of the Eu site in high-Tc cuprates. The average orientation angles obtained from fittings of the Mössbauer spectra were in accordance with the results from the x-ray-diffraction measurements.
  • Characterization of superconducting Bi2Sr2Can-1CunO4+2n phases with 57Fe Mössbauer spectroscopy
    (1990) Tittonen, I.; Hietaniemi, J.; Huttunen, J.; Lindén, J.; Katila, T.; Karlemo, T.; Karppinen, Maarit; Niinistö, L.; Ullakko, K.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In order to study the dependence of superconductivity on sample preparation and the influence of cationic doping, Bi2Sr2Can−1CunO4+2n samples were prepared using various sintering temperatures and nominal starting compositions. After each sintering the samples were characterized with x-ray-diffraction and magnetic-ac-susceptibility measurements. In all cases the corresponding doped57 samples were also made for Mössbauer measurements. The Mössbauer measurements usually show three quadrupole components arising from the different phases in the samples. The site assignment of Fe57 is discussed.
  • Characterization of superconducting Bi2Sr2Can-1CunO4+2n phases with Fe57 Mössbauer spectroscopy
    (1990) Tittonen, I.; Hietaniemi, J.; Huttunen, Juhani; LINDÉN, J.; Katila, T.; Karlemo, T.; Karppinen, M.; Niinistö, L.; Ullakko, K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In order to study the dependence of superconductivity on sample preparation and the influence of cationic doping, Bi2Sr2Can-1CunO4+2n samples were prepared using various sintering temperatures and nominal starting compositions. After each sintering the samples were characterized with x-ray-diffraction and magnetic-ac-susceptibility measurements. In all cases the corresponding doped57 samples were also made for Mössbauer measurements. The Mössbauer measurements usually show three quadrupole components arising from the different phases in the samples. The site assignment of Fe57 is discussed.
  • Europium substitution effects in superconducting YBa2Cu4O8 synthesized under one atmosphere oxygen pressure
    (1994) Lindén, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Kareiva, A.; Karppinen, Maarit; Niinistö, L.; Valo, J.; Leskelä, M.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Y1−xEuxBa2Cu4O8 powder samples, with x=0, 0.25, 0.5, 0.75, and 1.0, were synthesized at ambient pressure using either an acetate-tartrate sol-gel method or a LiF flux process. The lattice parameters and purity of the samples were checked using X-ray diffraction. The superconducting transition was monitored by magnetic-susceptibility measurements. Replacing yttrium with europium increased the unit-cell volume, decreased the orthorhombicity (b/a) and the critical temperature. The hyperfine interactions at the europium site were studied by Eu151 Mössbauer spectroscopy. The complete quadrupole Hamiltonian of the 21.5-keV γ transition of Eu151 was successfully applied in the analyses of the Mössbauer spectra. The Mössbauer parameters obtained were found to resemble those measured for the EuBa2Cu3Cu3O7−δ (1:2:3) system. It was demonstrated that magnetic alignment of the crystallites could not be obtained with an 11.7-T field, contrary to the 1:2:3 and other high-Tc systems. The magnetic susceptibility for 1:2:4 single crystals appears to be isotropic.
  • Europium-based high-temperature superconductors studied by x-ray diffraction and 151Eu Mössbauer spectroscopy
    (1992) Lindén, J.; Hietaniemi, J.; Ikonen, E.; Lippmaa, M.; Tittonen, I.; Katila, T.; Karlemo, T.; Karppinen, Maarit; Niinistö, L.; Ullakko, K.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Isotropic powders and magnetically aligned crystallites of EuBa2Cu3O7−δ (1:2:3) and europium-doped Bi2Sr2CaCu2O8 (2:2:1:2) were studied by means of x-ray diffraction and Eu151 Mössbauer spectroscopy. The degree of crystallite orientation of the samples and the values of the lattice constants were determined by x-ray diffraction. The Mössbauer spectra were analyzed considering the full hyperfine Hamiltonian of the nuclear states of the 21.5-keV γ transition. The Mössbauer hyperfine parameters obtained from the superconducting and semiconducting phases are presented. A small change is seen in the Eu151 isomer shift when the oxygen deficiency δ of the 1:2:3 compound is varied. The shift can be explained by a decrease in the s-electron density due to lattice expansion. The changes in the oxidation state of the copper atoms with varying δ were determined from the Mössbauer data: The Cu(2) atoms retain their oxidation state, whereas the Cu(1) atoms adjust their valence according to the value of δ. In the 2:2:1:2 samples, the Eu concentration clearly affected the value of the electric-field gradient at the Eu nucleus. Using a standard procedure, magnetically aligned 2:2:1:2 samples were prepared. The preferred direction of the crystal c axis changed from parallel to perpendicular alignment with the external magnetic field, when the Eu concentration exceeded 20% of the Ca atoms.
  • Precise determination of the hyperfine parameters of europium in multifluorite perovskites by 151Eu Mössbauer spectroscopy
    (1994) Lindén, J.; Lippmaa, M.; Miettinen, J.; Tittonen, I.; Katila, T.; Karppinen, Maarit; Niinistö, L.; Nara, A.; Yamauchi, H.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    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 studied by Eu151 Mössbauer spectroscopy. The work was motivated by the search for new superconducting phases. This homologous series is based on the YBa2Cu3O7−δ (1:2:3) structure. The samples used in the Mössbauer measurements consisted of crystallites with random orientation and grain oriented crystallites. The texture of oriented samples was analyzed by x-ray diffraction. The complete quadrupole Hamiltonian of the 21.5-keV γ-transition of Eu151 was successfully applied in the analyses of all the Mössbauer spectra. In samples having n≤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 the latter samples was made possible by simultaneous fitting of three spectra, corresponding to three different crystal orientations of the same specimen. This fitting scheme also enables more precise determination of the hyperfine parameters in the n=2 samples. In these samples an electric field gradient (EFG), with a large negative-valued main component (Vzz) 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 Vzz components of opposite signs. The negative Vzz value was attributed to the rare-earth site adjacent to the CuO layer. This site, found in all samples of the series, corresponds to the rare-earth site of the 1:2:3 system.