H. Engelmann

Influence of Fe-substitution on the high-Tc superconductivity

Abstract The influence of Fe-substitution on the high-Tc superconductor YBa2Cu3O7 was studied by means of d.c.-magnetic susceptibility and X-ray powder diffraction. Fe was found to exist in the paramagnetic high-spin state. Superconductivity was found in the samples with up to 20% Fe. A 4% Fe-substitution induces a tetragonal structure at room temperature with almost no effect on Tc. The very high Tc of 89 K of this fully tetragonal phase indicates that the linear CuO chain might not be critical for the superconductivity.

Cobalt-silicide structures studied by Mössbauer spectroscopy

Mössbauer spectroscopy of cobalt-silicides proved the existence of three stoichiometric phases: CoSi2, CoSi and Co2Si. The values of the hyperfine parameters indicated covalent bonds between Co and Si in all phases and could be understood on the basis of their crystallographic structure and of theoretical predictions for the electronic structure. For CoSi2, anomalous spectra were obtained, the origin of which was not clear. The different spectra of the cobalt-silicides permit the prospective use of Mössbauer spectroscopy for the study of formation and structure of Co/silicide/silicon interfaces.

Investigation of the ferroelectric phase transition in LiNbO3: Fe by Mössbauer spectroscopy

Abstract The ferroelectric phase transition in LiNbO3: Fe has been investigated by means of Mossbauer spectroscopy. The recoilless fraction of the γ-rays showed a strong anisotropy, reflecting large amplitudes of the Fe-ions along the hexagonal c-axis. Since a strong coupling of the Fe2+-ions to the Li-sublattice could be proved, the observed behaviour of the Fe cations supports the model proposed by Abrahams et al. [1], according to which above Tc, the Li-ions occupy equivalent non-centrosymmetric sites on either side of the oxygen ion planes with equal probability, while the Nb-ions shift to centrosymmetric positions.

On the structure of amorphous LiNbO3

Amorphous LiNbO3 doped with 0.22 at.% Fe2O3 (90.7% enriched in 57Fe) was prepared using the twin roller quenching technique. Mossbauer measurements indicate that the short range order structure is similar to that of the crystalline phase. Positron annihilation experiments revealed an increase of the positron lifetime in the amorphous phase which may result from an increase of free volume.

A Mössbauer and ESR study of LiNbO3-Fe2O3 for low Fe2O3 concentrations

Samples of the system LiNbO3-Fe2O3 prepared by water quenching and by the double-roller quenching method in the range up to 24 mol% Fe2O3 were investigated by Mössbauer and ESR spectroscopy. In the water quenched samples up to 11 mol% Fe2O3 only the Fe3+ and the Fe2+ valence states could be detected. The Fe2+ concentration decreased with increasing Fe2O3 content. Above 11 mol% Fe2O3 magnetically split Mössbauer spectra indicated the presence of Fe2O3 clusters. The isomer shift values of Fe3+ as a function of Fe2O3 concentration showed jumps at 6 and 11 mol% Fe2O3, whereas no significant changes could be detected in the quadrupole splitting values. The ESR data already exhibited the existence of isolated Fe3+ ions and of clusters with Fe-Fe distances less than 8 Å for the lowest Fe2O3 concentration. The cluster signal intensity increased with increasing Fe2O3 content. The roller quenched samples showed increased Fe2+ concentration as compared to the water quenched samples, which suggests that slow quenching results in iron oxidation and cluster formation. For low Fe2O3 concentrations a valence state change Fe3+⇄Fe2+ can easily be obtained by heat treatments in various atmospheres, whereas for higher Fe2O3 contents (9.8 mol%) precipitations ofα-Fe (in reducing atmosphere) and Fe2O3 (in air) could be observed in addition to the valence state changes of a remaining part of dissolved Fe ions. On the basis of the obtained results a model was suggested for the unusual behaviour of the lattice parameters observed in LiNbO3-Fe2O3.

The green stain on the bark of plane trees — A new probe for studying the iron content in air pollution

The origin of iron complexes detected several years ago in the green stain on the bark of plane trees was not clear up to now. Using Mössbauer spectroscopy, we were able to show that these iron complexes result from air pollution deposits. Since all iron compounds detected in air pollution so far are also present in green stain on the bark of plane trees in high concentrations, it seems to be a very effective indicator of air pollution.

Investigation of anomalous transitions in LiNbO3 by Mossbauer spectroscopy

According to Ismailzade et al.1 LiNbO3, shows eleven anomalies in lattice parameters below the ferroelectric phase transition Tc = 1153°C. A later detected anomaly near 75°C is the most intensively investigated. An easy cracking of LiNbO3, on cooling from 80 to 60°C is reported by Xu Ziran et a1.2 Additional anomalies in birefringence,2 ultrasonic attenuation3 and Mossbauer parameters4 have been observed. Although a phase transition is suggested to occur at 75°C by some author,2, 3 the behaviour of LiNbO3 at 75°C is still not completely understood.2, 5 Encouraged by interesting results obtained in initial Mossbauer investigations of Hsia et al.4 we undertook detailed studies of the anomalies near 75 and 125°C in LiNbO3.

Mössbauer study of proton-exchanged LiNbO3:Fe

Topotactic proton exchange (Li against H) can be achieved by treating LiBnO3 with appropriate acids. In order to investigate the effect of proton exchange on Fe-impurities we studied LiNbO3:Fe powder material treated in sulphuric acid and LiNbO3:Fe single crystals treated in benzoic acid by Mössbauer spectroscopy. During the topotactic ion exchange only the Li-ions are exchanged for protons, whereas the Fe-impurities are retained in the material.

Mössbauer effect of57Co in LiTaO3 — Effects of a variety of heat treatments

LiTaO3 single crystals were doped with57Co. Heat treatments of the crystals in the various atmospheres (Ar, Ar/H2 etc.) very sensitively influenced the charge states of iron formed after the EC of57Co. Fe2+ Fe3+ and metallic clusters are formed depending on the treatments in the various atmospheres. The observed effects were found reversible by changing the reducing and oxidising atmospheres.

Structural and magnetic properties of amorphous ferric molybdate compared to crystalline ferric molybdate

The two component system Fe2 (MoO4)3. MoO3 denoted as CFM (crystalline ferric molybdate) has been transformed to the amorphous state by application of shearing stress under high pressure. The transition from amorphous ferric molybdate (AFM) to CFM can be achieved by air annealing of AFM at T>550 K. A detailed study of the crystallization process has been performed by Mössbauer, X-ray and microcalorimetric measurements. A proposal for the “structure” of the amorphous state and for the crystallization process is made. Below the Néel temperature (TN=13 K) CFM orders weakly ferrimagnetic, whereas the magnetic ordering of AFM occurs below 4.2 K with a still unknown magnetic phase.