J. Bánkuti

Metastability in EuBa2(Cu1-xSnx)3O7-y studied by 119Sn and 151Eu Mössbauer spectroscopy.

The metallic oxide compound system EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ (x = 0.05, 0.10, 0.20) was prepared for investigation with /sup 151/Eu and /sup 119/Sn Moessbauer spectroscopy. The best fits of the /sup 119/Sn Moessbauer lines reveal at least two different Sn/sup IV/ sites. The /sup 151/Eu spectra are characteristic for Eu/sup III/ state in these compounds. We have found a considerable difference between the shapes of /sup 119/Sn Moessbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior.

Metastability in EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ studied by /sup 119/Sn and /sup 151/Eu Moessbauer spectroscopy

The metallic oxide compound system EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ (x = 0.05, 0.10, 0.20) was prepared for investigation with /sup 151/Eu and /sup 119/Sn Moessbauer spectroscopy. The best fits of the /sup 119/Sn Moessbauer lines reveal at least two different Sn/sup IV/ sites. The /sup 151/Eu spectra are characteristic for Eu/sup III/ state in these compounds. We have found a considerable difference between the shapes of /sup 119/Sn Moessbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior.

The effect of Pr substituted in Eu1−xPrxBa2Cu3O7−δ and EuBa2−xPrxCu3O7−δ perovskites

Abstract Eu1−xPrxBa2Cu3O7−δ (x=0.0, 0.1, 0.5, 0.7, 1.0) and EuBa2−xPrxCu3O7−δ (x=0.0, 0.1, 0.3, 0.5, 0.7) perovskites have been synthesized and investigated by means of 151 Eu Mossbauer spectroscopy, X-ray diffractometry, resistivity and susceptibility measurements. We have found that Pr can be introduced into the Ba site in EuBa2Cu3O7−δ high temperature superconductor. Our results show that EuBa2−xPrxCu3O7−δ is a superconductor for x≤0.7.

57Fe,119Sn and151Eu Mössbauer study of EuBa2(Cu1−x−y119Snx57Fey)3O7−δ superconductor

Abstract151Eu,119Sn and57Fe Mössbauer spectroscopy was used to study a EuBa2(Cu1−x−ySnx Fev)3O7−δ superconductor in which sites were replaced by all three Mössbauer isotopes in the same sample in order to get information about the site preferences and structural changes. We have found changes in the Mössbauer parameters of the57Fe,119Sn and151Eu spectra compared to those recorded in the separately substituted EuBa2Cu3O7−δ, EuBa2(Cu1−xSnx)3O7−δ, YBaCu3−xFexO7−δ YBaCu3−xSnxO7−δ materials. These results can be interpreted as a consequence of the site preferences and the changes in the structure due to the presence of other additional substitutional elements.

High-Tc Superconductivity in La-Ba-Cu-O and Y-Ba-Cu-O Compounds

A high-critical-temperature superconducting transition in multiple-phase La-Ba-Cu-O and Y-Ba-Cu-O systems has been found with the aid of electrical and magnetic measurements. The possible causes of superconductivity is also considered. Among them the change of Fermi-surface topology is the most appreciated.

First observation of structural changes around the Tc in TlBaCaCuO4.5+y superconductor studied by57Fe Mössbauer spectroscopy

Abstract57Fe Mössbauer spectroscopy was used to study57Fe doped high Tc TlBaCaCuO4.5+y superconductor between 70 and 295 K. Significant anomalous changes in the shape of the Mössbauer spectrum were observed around the Tc /105–120 K/. This experimental fact could be reproduced. Our finding can be associated with structural changes taking place around the Tc in TlBaCaCuO4.5+y superconductor.

119Sn and151Eu Mössbauer study of EuBa2(Cu1-xSnx)3O7-y perovskites

Eu and Sn Mössbauer spectroscopy was used to study EuBa2(Cu1-xSnx)3O7-y metallic oxides. The spectra are characteristic for EuIII and SnIV states in all cases. The existence of at least two different Sn states was determined by decomposition of the Sn Mössbauer spectra. These Sn sites can be associated with nonequivalent Cu sites replaced by Sn atoms in the perovskite type lattice. Anomalous changes were observed in the isomer shifts and area fractions in the spectra measured at room temperature and at 77 K. It can be interpreted assuming low temperature phase transformation and phonon softening. Time dependent changes were found in the Sn spectra recorded at 77 K in the case of highest Sn concentration. The observed changes are consistent with a transitional stage of the low temperature phase transformation.

Design and construction of a small-scale model of a high-temperature superconducting magnetically levitated vehicle

The design of a small-scale model of a magnetically levitated vehicle is presented in the paper. The model has been constructed on the basis of the calculation of the magnetic field and forces between permanent magnets and high temperature superconducting materials prepared by different fabrication techniques. The fabrication of the superconductor play a fundamental role in designing the vehicle since large enough levitation forces can be achieved only with superconductors that have large pinning forces. The fabrication processes, the design with a special emphasis on the calculation of the magnetic field and model construction are reported.

Development of superconducting laboratory magnet series

A laboratory superconducting magnet series was developed for the research of the magnetic phenomena of type II superconductors. The magnetic field of the different members of this series comprises from 1.5 kOe to 72.6 kOe. Our aim in the constructions was to minimise the volume of solenoids to avoid the large helium consumption, and - in some cases - to reach high homogeneity by optimalised design using a graphical interpolation technique with computer. There are magnets with special working chambers for high pressure investigation and for orientation of chemical reactions.

Mössbauer study of Tl containing high Tc superconductors

Mössbauer spectroscopy, X-ray diffractometry, ESR and electrical resistivity measurements, were used to study highTc (above 100 K) superconducting materials in order to get information about the effect of the preparation circumstances as well as about the anomalous behaviour in57Fe and119Sn doped TlBaCaCu(57Fe)O4.5+δ and TlBaCuCu(119Sn)O4.5+δ superconductors. We have found that the Mössbauer parameters strongly depend on the preparation circumstances. In many cases the presence of Mössbauer lines of nonsuperconducting oxide phases indicated that the iron and tin could not entirely built in into the lattice of superconductor. Four valence state tin and four valence state iron sites were identified in the superconducting phases. We have found anomalous temperature dependent changes in the57Fe Mössbauer spectra of TlBaCaCu(57Fe)O4.5+δ samples around theTc between 105–135 K. In this temperature range the total area of the spectrum temporary increases.