J. Stanek

Hyperfine interactions of iron implanted into aluminium

Stable57Fe implanted into Al at energies of 20 to 70 keV and doses of 1014 to 2·1017 ions/cm2 was studied with conversion electron Mössbauer spectroscopy at room and liquid nitrogen temperatures. Spectra composed of a single line and a doublet were observed. Similarly as in the splat-quenched FeAl alloys iron monomers and iron associations, mostly dimers, are observed. The isomer shifts of both components differ considerably and do not change with iron concentration. The splitting of the doublet increases with iron concentration, the increase being reproduced by computer simulations of electric field gradients in lattices with a random distribution of charge defects. The observed probability of formation of iron associates is higher than in random systems, especially at high iron doses.

Tuning of Charge Transfer Assisted Phase Transition and Slow Magnetic Relaxation Functionalities in {Fe9–xCox[W(CN)8]6} (x = 0–9) Molecular Solid Solution

Precisely controlled stoichiometric mixtures of Co(2+) and Fe(2+) metal ions were combined with the [W(V)(CN)8](3-) metalloligand in a methanolic solution to produce a series of trimetallic cyanido-bridged {Fe(9-x)Co(x)[W(CN)8]6(MeOH)24}·12MeOH (x = 0, 1, ..., 8, 9; compounds 0, 1, ..., 8, 9) clusters. All the compounds, 0-9, are isostructural, and consist of pentadecanuclear clusters of a six-capped body-centered cube topology, capped by methanol molecules which are coordinated to 3d metal centers. Thus, they can be considered as a unique type of a cluster-based molecular solid solution in which different Co/Fe metal ratios can be introduced while preserving the coordination skeleton and the overall molecular architecture. Depending on the Co/Fe ratio, 0-9 exhibit an unprecedented tuning of magnetic functionalities which relate to charge transfer assisted phase transition effects and slow magnetic relaxation effects. The iron rich 0-5 phases exhibit thermally induced reversible structural phase transitions in the 180-220 K range with the critical temperatures being linearly dependent on the value of x. The phase transition in 0 is accompanied by (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) charge transfer (CT) and the additional minor contribution of a Fe-based spin crossover (SCO) effect. The Co-containing 1-5 phases reveal two simultaneous electron transfer processes which explore (HS)Fe(II) W(V) ↔ (HS)Fe(III) W(IV) CT and the more complex (HS)Co(II) W(V) ↔ (LS)Co(III) W(IV) charge transfer induced spin transition (CTIST). Detailed structural, spectroscopic, and magnetic studies help explain the specific role of both types of CN(-)-bridged moieties: the Fe-NC-W linkages activate the molecular network toward a phase transition, while the subsequent Co-W CTIST enhances structural changes and enlarges thermal hysteresis of the magnetic susceptibility. On the second side of the 0-9 series, the vanishing phase transition in the cobalt rich 6-9 phases results in the high-spin ground state, and in the occurrence of a slow magnetic relaxation process at low temperatures. The energy barrier of the magnetic relaxation gradually increases with the increasing value of x, reaching up to ΔE/kB = 22.3(3) K for compound 9.

Mössbauer studies of the non-heme iron and cytochrome b559 in a Chlamydomonas reinhardtii PSI- mutant and their interactions with α-tocopherol quinone

Spin and valence states of the non‐heme iron and the heme iron of cytochrome b 559, as well as their interactions with α‐tocopherol quinone (α‐TQ) in photosystem II (PSII) thylakoid membranes prepared from the Chlamydomonas reinhardtii PSI− mutant have been studied using Mössbauer spectroscopy. Both of the iron atoms are in low spin ferrous states. The Debye temperature of the non‐heme is 194 K and of the heme iron is 182 K. The treatment of α‐TQ does not change the spin and the valence states of the non‐heme iron but enhances the covalence of its bonds. α‐TQ oxidizes the heme iron into the high spin Fe3+ state. A possible role of the non‐heme iron and α‐TQ in electron flow through the PSII is discussed.

FeII Spin‐Crossover Phenomenon in the Pentadecanuclear {Fe9[Re(CN)8]6} Spherical Cluster

The self-assembly of iron(II) ions with rare octacyanidorhenate(V) metalloligands in a methanolic solution results in the formation of a nanometric pentadecanuclear {Fe(II) 9 [Re(V) (CN)8 ]6 (MeOH)24 }⋅10 MeOH (1) molecule with a six-capped body-centered cubic topology. The cluster demonstrates a thermally-induced spin-crossover phase transition at T1/2 =195 K which occurs selectively for a single Fe(II)  ion embedded in the center of a cluster core.

Conversion electron Mössbauer study of LiF implanted with 57Fe ions

Abstract Lithium fluoride crystals implanted with 57Fe ions at doses from 5×1015 at/cm2 to 6×1016 at/cm2 have been studied with conversion electron Mossbauer spectroscopy. It was found that iron enters the implanted zone in three well defined charge states: Fe3+, Fe2+ and Fe0 (metal). At low doses the relative fractions of Fe3+ and Fe2+ are close to 80% and 20%, which corresponds to the respective ionic fractions in an iron doped LiF sample obtained by vacuum co-deposition. With the increase in the implantation dose the Fe2+ and Fe0 fractions increase to about 40% and 30%, respectively. Annealing in vacuum results in a growth of metallic iron precipitates, whereas annealing in the presence of oxygen leads to precipitation of superparamagnetic particles of some, still unidentified, ferric compound. The Mossbauer data were related to the role of F- and F2-centres observed by optical absorption measurements. Good agreement has been achieved between the quadrupole splitting data for Fe3+ in fluorides LiF, NaF and KF and the calculated values in frames of the model of V−-Fe3+-V− defect clusters presented in our earlier work.

Temperature‐dependent conversion electron Mössbauer measurements of 57Fe implanted in silicon and germanium

Conversion electron Mossbauer spectra of 3 at% Fe implanted in silicon and germanium are measured at temperatures between 78 and 550 K. Helium counters of electrons are used in back-scattering geometry. The results indicate a temperature-independent s-electron density at the iron nuclei. A rather small temperature dependence of the electric field gradient is observed and is interpreted in terms of the T3/2-relationship. The possible location of iron implanted in amorphous silicon and germanium is discussed. Es werden die Elektronenkonversions-Mosbauerspektren von 3 At% Fe, das in Silizium und Germanium implantiert wurde, bei Temperaturen zwischen 78 und 550 K gemessen. Heliumelektronenzahler werden in Ruckstreugeometrie benutzt. Die Ergebnisse zeigen eine temperaturunabhangige s-Elektronendichte am Eisenkern. Eine ziemlich kleine Temperaturabhangigkeit des elektrischen Feldgradienten wird beobachtet und mit der T3/2-Beziehung interpretiert. Die mogliche Lokalisierung des implantierten Eisens in amorphem Silizium und Germanium wird diskutiert.

Conversion electron Mössbauer spectroscopy of 197Au

Abstract The Mossbauer spectra of 77.3 keV γ-transition in 197Au have been measured for the first time in the conversion electron scattering mode, with the use of a channel electron multiplier at low temperatures. The high magnitude of the resonant effect obtained makes the investigations of various phenomena in very thin gold films feasible. The application of the technique in studying other high-energy Mossbauer transitions is also possible.

Mixed valence state in ironporphyrin aggregates

In biological systems, metalloporphyrins play a central role in energy and electron transfer process. Our aim is to understand the influence of ligands and iron coordination of ironporphyrin on the electron transfer. The lyophilized ironporphyrin, enriched in 57Fe up to 90% has been studied by Mössbauer spectroscopy between 2.8 and 313 K. Above room temperature the bounded diffusion of the ferric iron was observed. Below 293 K a part of iron appears in mixed Fe+3<==>Fe+2 valence state with 10 meV activation energy for the electron trapping. Below 4 K a part of iron shows magnetic ordering with a broad distribution of the hyperfine field. The results are discussed in terms of metalloporphyrin aggregation process.

Molecular dynamics and local electronic states of Sn and Fe in metallocytochrome and metalloporphyrin

In Sn-porphyrin tin appears in the Sn4+ state while in Sn-cytochrome c Sn4+ and Sn2+ states are observed. In Fe-porphyrin iron exhibits temperature-dependent mixed Fe2+-Fe3+ valency. In Fe-cytochrome iron is in the Fe3+ state. The bounded diffusion is observed in Fe-porphyrins above 300 K. The temperature dependence of the metal mean square displacement in cytochrome c and in Sn-porphyrin shows a deviation from the Debye model, which may be explained either by a new vibrational degree of freedom occurring above 150 K or by anharmonicity of the usual vibrations.

Channeltron detecting system for conversion electron Mo¨ssbauer spectroscopy at high temperatures

Abstract A vacuum apparatus suitable for measurements of conversion electron Mo¨ssbauer spectra at temperatures up to 1200 K is presented. The apparatus allows CEMS measurements and in situ heat treatment of the samples under ultra-high vacuum conditions. The detecting system is based on the channel electron multiplier used in a close-to-sample geometry. A special technique of suppressing the background due to thermally activated electrons and ions is described. Examples of measurements with a metallic iron foil above the Curie point and with crystals implanted with 57 Fe ions are given.