John G. Stevens

Mössbauer studies of the superconducting cobalt/nickel-doped BaFe2As2. Whither go the injected electron(s)?

Mössbauer studies of cobalt- and nickel-doped BaFe(2)As(2) show that the s-electron density at the (57)Fe nuclei, as measured by the isomer shift, is the same as that for the parent BaFe(2)As(2). Apparently, the electron population of the d shell, which shields the s-electron density at the nuclei, remains unchanged. We invoke the involvement of p-orbital hybridization with the d orbital in Fe-As bonding. Furthermore, the shrinkage of the lattice on substitution enhances the As-As sp hybridization, providing a path for the migration of additional electrons. The proposed mechanism is consistent with Hall coefficient and thermoelectric effect measurements.

Documentation and evaluation of Mössbauer data for minerals

More than 2,500 Mössbauer spectroscopic studies on minerals have been published since 1960. These papers contain approximately 8,000 sets of Mössbauer mineral data on at least 400 different minerals. This information has been compiled into a database that includes isomer shifts, quadrupole splittings, and magnetic hyperfine interactions at room, liquid nitrogen, and liquid helium temperatures. The database provides a profile of the Mössbauer research performed to date on minerals, including type and locality of minerals investigated, significance of the studies and results, and location of the research facilities. The data and information are available both in printed and magnetic form.With the establishment of this resource, the Mössbauer Effect Data Center has begun a systematic evaluation of the Mössbauer mineral data. It is anticipated that this resource can be used for the identification of minerals as powder X-ray diffraction data is.

Phyllosilicates: A Mössbauer evaluation

Based on the Mössbauer data of minerals extracted from theMineral: Data Handbook, a consistent set of best values for the isomeric shifts and the quadrupole splittings for ferrous and ferric ions in the cis and trans sites of phyllosilicate is given. The parameters, in particular the quadrupole splitting values, can be used in an analogous way for the identification of minerals as the Hanawalt and Fink methods of powder x-ray diffraction data where each mineral is identified by a set of unique Mössbauer data.

A review of Mössbauer spectroscopic studies of ancient pottery

Mössbauer spectroscopy has proven itself to be invaluable to archaeologists by providing a means to classify pottery and to provide information on particular providences for various ancient pottery finds. The original firing atmosphere can often by deduced from the ratio of Fe2+ to Fe3+. The change in the quadrupole splitting and the magnetic hyperfine splittings allow for the determination of the original firing temperatures. Ancient pottery samples from many cultures have been studied and a number of general conclusions are possible.

Formation of Magnetic FexOy/Silica Core-Shell Particles in a One-Step Flame Aerosol Process

In this study, iron silicon oxide particles were generated in a one-step flame assisted spray pyrolysis (FASP) process using H2/air or H2/O2 diffusion flames. A colloidal precursor solution was used, which contained dissolved iron nitrate and stably suspended silica nanoparticles. H2/air flames resulted in magnetic FexOy/silica core-shell particles. There was a correlation between particle size and particle structure; particles larger than 500 nm had the core-shell structure, but smaller particles had non-core-shell structures. H2/O2 flames only resulted in nanoparticles that had non-core-shell structures. The core-shell particles had a iron oxide core that was hermetically enclosed in a uniform silica shell with a typical thickness of approximately 100 nm; they were superparamagnetic with a room-temperature saturation magnetization greater than 24 emu/g. Temperature history of the particles may be used to explain the correlation between flame type and particle structure. The correlation between particle size and structure may be due to size-dependent thermodynamic stability of the structures, or kinetics of heat and mass transfer. The results from this study suggest that micrometer sized iron oxide silica core-shell magnetic particles could be generated from a one-step flame aerosol process, but FexOy/silica nanoparticles (< 100 nm) with the core-shell structure cannot be generated in a one-step flame aerosol process.

Structural Investigations of Chitin and Chitosan Complexed with Iron or Tin

Chitin (N-acetyl-glucosamine) and its derivative chitosan (glucosamine) bind with most transition and main group metals, including iron and tin. Using 57Fe and 119Sn Mössbauer Spectroscopy it is determined that an oxidation reaction occurs during the metal uptake. Data also supports a structure with more than one metal bonding site and shows the ability of the chitin and chitosan polymers to bind large concentrations of iron.

Mössbauer study of the Jilin and Xinyang meteorites

The Xinyang and Jilin meteorites were investigated using Mössbauer spectroscopy. In addition to troilites and silicates, the Jilin meteorite contains taenite and kamacite, while the Xinyang meteorite contains kamacite only. The Mössbauer data of these two meteorites confirm them as ordinary H chondrites. The Mössbauer parameters can be interpreted by a model based on the cooling history of these meteorites.

Ferrihydrite modification by boron doping

Modification of ferrihydrite nanoparticles with boron is investigated using Mössbauer spectroscopy and powdered X-ray diffraction. These materials were heated to temperatures up to 600°C. The addition of the boron in the fabrication of ferrihydrite nanoparticles raises the transition temperature of the phase transformation of ferrihydrite to hematite. Evidence is that the boron does not penetrate the iron oxihydroxide structure, but attaches itself to the surface of the particles.

The role of γ-iron nanoparticulates in the growth of carbon nanotubes

Carbon nanotubes (∼200nm diameter) are grown by chemical vapor deposition using catalytic iron particles. Mossbauer spectroscopy enables differentiation among relatively large Fe3C, α-Fe, and nanosized superparamagnetic fcc γ-Fe particles. The antiferromagnetic configuration of γ-Fe nanoparticles yields a significant fraction of uncompensated spins, producing a weak ferromagnetism that allows estimation of size (2–3nm) via magnetization in zero field versus variable field cooling. This property of γ-Fe nanoparticles has not been previously employed. We propose that the surfaces of 200nm iron carbide particles are covered with nanosized γ-Fe and graphitized carbon that participate in the catalytic growth of nanotubes.

Effects of fluorine substitution in superconducting Tl2Ba2CuO6±δ

Abstract The fluorine substituted thallium based 2 2 0 1 phase samples were fabricated with starting compositions 0.95Tl 2 O 3 + 0.1SnO 2 + (2− x )BaO 2 + x BaF 2 + CuO. Mossbauer and X-ray spectra were used as a study of the role played by fluorine. All samples show a near 2 2 0 1 pure phase. The best fluorine substituted sample has T c (onset) of 100 K and the T c (R = 0) of 97 K. The Mossbauer absorption area varies, parallel to T c , with the fluorine content and reaches a maximum absorption near x = 0.6, which indicates that fluorine could have replaced oxygen in the crystal lattice sites and thus does not rule out the presence of a phonon superconducting mechanism.