R.C. Thiel

155Gd Mössbauer measurements on the ceramic superconductor GdBa2Cu3O7

Abstract 155Gd Mossbauer effect measurements have been performed on GdBa2Cu3O7 both in the superconducting orthorhombic and in the semiconducting tetragonal phase. The observed isomer shifts are similar to those found in insulating Gd-transition metal-oxides, indicating that there is no conduction electron density within the Gd layers Below the temperature at which the specific heat shows a sharp peak both phases show magnetic ordering. The Gd moment is found to be parallel to the c-axis. Dipolar calculations show that the dipolar field at the Cu sites is negligibly small and that the magnetic ordering of the Gd moments is probably of the 2-d Ising type.

Some doubts about Fe substitution in ReBa2Cu3O7−δ

Abstract Mossbauer spectra have been obtained for YBa2Cu2.94Fe0.06O7−δ and GdBa2Cu2.94Fe0.06O7−δ at various temperatures down to 0.6 K. The low temperature measurements show four magnetic hyperfine split spectra, two of which are not compatible with Fe3+; combined with I.S. data, these spectra are indicative of the Fe4+ state. A comparison is made with the Mossbauer parameters for BaFeO3−x. The transition from paramagnetic to magnetically split spectra indicates that much of the 57Fe substituted into these high-Tc materials is inhomogeneously distributed over the Cu sites. Because of the likelyhood that the high oxidation state of the iron will effect the local oxygen surroundings, the usefulness of 57Fe substitution into these materials is at best questionable.

Physical properties of metal cluster compounds VI: The influence of the ligands on the 197Au Mössbauer spectra of three different Au55 molecular clusters

Abstract The results of Mossbauer Effect measurements on the methylated gold molecular cluster compound Au55(P(C6H4CH3)3)12Cl6 and the water soluble compound Au55(PPh2C6H4SO3Na·2H2O)12Cl6 are compared to those on the original gold cluster compound Au55(PPh3)12Cl6. A new procedure for the determination of the f-factor of the source is presented. The differences between the spectra for the three versions of this molecular cluster compound are discussed. An attempt is made to draw some preliminary conclusions.

Heat capacity of thick-film resistor thermometers and pure RuO2 at low temperatures

Abstract The heat capacity of commercial thick-film chip resistors has been measured in the temperature range 0.1–2K. Three different types of resistors used as low temperature thermometers were investigated. The heat capacity of all samples shows a Schottky-type anomaly with the maximum at 0.4K amounting to 1μJ K −1 for the largest unit. We also report the data on the specific heat of a thermometer component, RuO 2 , and show that it has no anomalous behaviour. The peak in heat capacity of these resistors puts limitations on using them as thermometers for low temperature calorimeters.

121Sb Mössbauer measurements on BaPb1−xSbxO3 below the superconducting transition temperature

Abstract We report Mossbauer experiments on the compounds BaPb 1− x Sb x O 3 with x =0.10, 0.25, and 0.30. We find no evidence for a mixed valence state of the Sb ions. The valence of Sb as determined on the timescale of the Mossbauer experiment is shown to be 5+. There is no visible change in the isomer shift on passing below T c . Some suggestions are made for the role that Pb plays in the superconducting and insulating compounds.

Field-dependent Mössbauer relaxation study of domain walls in the quasi 1-d antiferromagnet RbFeCl3.2H2O

Abstract Domain walls (narrow solitons) in the Ising-type quasi 1-d antiferromagnet RbFeCl3.2H2O have been studied by means of Mossbauer relaxation experiments in magnetic fields strong enough (1.5 T) to suppress the 3-d long-range ordering. Excellent agreement is obtained with the Blume - Tjon model, assuming stochastic relaxation due to the propagation of wall- pairs . In still higher fields (6 T) the excitation of walls is found to be suppressed by the generation of a staggered field on the (slightly canted) antiferromagnetic chains.

A reexamination of theI.S. results for the cluster compound Au55(PPh3)12Cl6

The observed lowI.S. values [1] for the four distinct gold sites in the cluster compound Au55(PPh3)12Cl6 have been explained in terms of a decrease of the 6s electron density at the nucleus, due either to a lattice expansion or to a delocalization of 6s electrons over the ligand shell surrounding the gold core. Recent EXAFS measurements [2] indicate a single average distance between the gold atoms, about 3.5% less than for bulk gold. This not only excludes the lattice expansion hypothesis, it effectively increases the density of 6s electrons, making an explanation for the observedI.S. even more difficult.For an understanding of theI.S. values it is necessary to reconsider the probable occupation of the 5d orbitals within the framework provided by the XPS surface atom core level shifts developed by Citrin and Wertheim [3]. Partial confirmation has been found in preliminary XPS results [5]. The consequences for thef-factor and specific heat results [1] will also be examined.

Electronic structure and bonding of the metal cluster compound Au55(PPh3)12Cl6

We discuss the electronic structure, bonding and physical properties of the gold cluster compound Au55(PPh3)12Cl6. Results from our experimental measurements, including EXAFS, specific heat, Mössbauer, UV-visible and photoelectron spectroscopy, are combined with those of other work to form a consistent physical picture of the system. The bonding in Au55(PPh3)12Cl6 is much more delocalised and non-directional than in smaller gold cluster molecules. The Au55 cluster exhibits a substantial degree of metallic bonding, while displaying some of the characteristics of a discrete energy level spectrum.

Mössbauer relaxation and thermodynamic properties of spin cluster-triad in EuMg5

The existence of cluster of three-antiferromagnetically coupled spin S=7/2 in the EuMg5 is found to give rise to peculiar behaviour in the thermodynamic as well as the Mössbauer relaxation. The A.C. susceptibility, specific heat and the high field magnetization as well as the ME spectra are reported and interpreted in terms of a model in which the intra-cluster interaction is treated exactly while the inter-cluster interaction is approximated by a molecular field.