Masayuki Kurokuzu

Synchrotron radiation-based Mössbauer spectra of 174Yb measured with internal conversion electrons

A detection system for synchrotron-radiation (SR)-based Mossbauer spectroscopy was developed to enhance the nuclear resonant scattering counting rate and thus increase the available nuclides. In the system, a windowless avalanche photodiode (APD) detector was combined with a vacuum cryostat to detect the internal conversion (IC) electrons and fluorescent X-rays accompanied by nuclear de-excitation. As a feasibility study, the SR-based Mossbauer spectrum using the 76.5 keV level of 174Yb was observed without 174Yb enrichment of the samples. The counting rate was five times higher than that of our previous system, and the spectrum was obtained within 10 h. This result shows that nuclear resonance events can be more efficiently detected by counting IC electrons for nuclides with high IC coefficients. Furthermore, the windowless detection system enables us to place the sample closer to the APD elements and is advantageous for nuclear resonant inelastic scattering measurements. Therefore, this detection system...

Development of 151Eu Time-Domain Interferometry and Its Application for the Study of Slow Dynamics in Ionic Liquids

Time-domain interferometry (TDI) employing 151Eu nuclear resonant scattering was developed for the study of slow dynamics. We measured the relaxation times of the density correlation in super-cooled ionic liquid 1-butyl-3-methylimidazolium iodide (BmimI) by the developed TDI. We found that the temperature dependence of the relaxation times follows the Vogel–Fulcher–Tammann law and observed fragile behavior of BmimI. Furthermore, we discussed the potential of TDI for the study of slow dynamics.

Nuclear Resonance Vibrational Spectroscopy and DFT study of Peroxo-Bridged Biferric Complexes: Structural Insight into Peroxo Intermediates of Binuclear Non-heme Iron Enzymes†

Binuclear non-heme iron enzymes utilize O2 to catalyze a variety of reactions, including hydrogen atom abstraction, desaturation, electrophilic aromatic substitution, and so on. In most cases, their catalytic cycles begin with the reductive binding of O2 by biferrous centers to form high-spin antiferromagnetically coupled (AFC) peroxo-bridged biferric intermediates. These peroxo intermediates can either react with substrate or convert to more reactive high-valent species. Because of their transient nature, structural information must be deduced from spectroscopic data, which are rich for some peroxo intermediates, while for others too limited for geometric and electronic structural insight. The peroxo intermediate of W48F/D84E ribonucleotide reductase (RR), referred to as P, does exhibit distinct spectral features. These include electronic absorption (Abs) and resonance Raman (rR) spectra that are equivalent to those of cis m-1,2 end-on peroxo-bridged Fe2 model complexes, thus providing a basis for the computational model of P as a cis m-1,2 peroxo-bridged Fe2 species (with the (Glu)4(His)2 ligand set of this protein active site). However, P is not reactive and must convert to a second-peroxo-level intermediate P’ that does not have Abs spectral features for rR-based structural elucidation. For systems that do not have chromophores or are photoactive, nuclear resonance vibrational spectroscopy (NRVS) is an alternative to rR spectroscopy. NRVS is a synchrotron-based technique that probes vibrational side bands of Fe nuclear transitions. Its spectral intensity is determined by the amount of Fe displacement in each normal mode, thus allowing the specific investigation of the Fe active site with high sensitivity and without the limitation of the selection rules of rR spectroscopy. In this study, we establish the basis for the NRVS analysis of peroxo-bridged Fe2 intermediates, based on structurally well-characterized synthetic model complexes. We have measured the NRVS spectra of [Fe2(mOH)(mO2)(6Me2-BPP)2] + (1) and [Fe2(mO)(mO2)(6Me2-BPP)2] (2 ; Figure 1; 6Me2-BPP = N,N-bis(6methyl-2-pyridylmethyl)-3-aminopropionate). These complexes are cis m-1,2 peroxo-bridged species, the former with an additional hydroxo bridge and the latter with an oxo

Observation of Enhancement of the Morin Transition Temperature in Iridium-Doped α-Fe2O3 Thin Film by 57Fe-Grazing Incidence Synchrotron Radiation Mössbauer Spectroscopy

The Morin transition of a (0001)-oriented iridium-doped α-Fe2O3 thin film deposited on an Al2O3(0001) substrate was studied by 57Fe-grazing incidence synchrotron radiation Mossbauer spectroscopy (GISRMS). Temperature-dependent spectra proved that the iridium doping markedly enhanced the Morin temperature of the α-Fe2O3 thin film; the iron spin directions were perpendicular to the film plane at temperatures below 100 °C, while they were in-plane at temperatures above 150 °C. The antiferromagnetic ordering was maintained far above 400 °C. The results demonstrated the availabilities of 57Fe-GISRMS, which enables a very quick evaluation of the magnetism in antiferromagnetic ultrathin films at high temperatures.

Improvement of Efficiency of Time-Domain Interferometry Method Using Two Driven Nuclear Absorbers

A time-domain interferometry (TDI) method using the nuclear resonant scattering of synchrotron radiation enables us to study the microscopic slow dynamics of around 100 ns in supercooled liquids and soft matters. To improve the TDI efficiency, we developed a new TDI method by adding another driven nuclear absorber with a single-line Mossbauer spectrum to the conventional TDI method. By using the nuclear absorbers with the optimum thickness estimated by simulation, new TDI experiments were successfully performed and the efficiency was improved about two times that obtained by the conventional TDI method used thus far.

(61)Ni synchrotron radiation-based Mössbauer spectroscopy of nickel-based nanoparticles with hexagonal structure.

We measured the synchrotron-radiation (SR)-based Mössbauer spectra of Ni-based nanoparticles with a hexagonal structure that were synthesised by chemical reduction. To obtain Mössbauer spectra of the nanoparticles without 61Ni enrichment, we developed a measurement system for 61Ni SR-based Mössbauer absorption spectroscopy without X-ray windows between the 61Ni84V16 standard energy alloy and detector. The counting rate of the 61Ni nuclear resonant scattering in the system was enhanced by the detection of internal conversion electrons and the close proximity between the energy standard and the detector. The spectrum measured at 4 K revealed the internal magnetic field of the nanoparticles was 3.4 ± 0.9 T, corresponding to a Ni atomic magnetic moment of 0.3 Bohr magneton. This differs from the value of Ni3C and the theoretically predicted value of hexagonal-close-packed (hcp)-Ni and suggested the nanoparticle possessed intermediate carbon content between hcp-Ni and Ni3C of approximately 10 atomic % of Ni. The improved 61Ni Mössbauer absorption measurement system is also applicable to various Ni materials without 61Ni enrichment, such as Ni hydride nanoparticles.

Mössbauer Spectroscopy of La0.87Ca0.13FePO and LaFeAsO0.93F0.07 under External Magnetic Field and Nuclear Resonant Inelastic Scattering of La0.87Ca0.13FePO

An 57 Fe Mossbauer spectroscopic study of iron-based layered superconductors La 0.87 Ca 0.13 FePO and LaFeAsO 0.93 F 0.07 under external magnetic fields of up to 14 T was performed. Our results suggest that hyperfine fields induced by local magnetic moments are, if any, small and that the magnetic feature of Fe in La 0.87 Ca 0.13 FePO and LaFeAsO 0.93 F 0.07 is most likely to be paramagnetism with itinerant electrons. This feature is confirmed both above and below the transition temperature for LaFeAsO 0.93 F 0.07 . Furthermore, the temperature dependence of the iron-specific phonon density of states in La 0.87 Ca 0.13 FePO was measured by using the nuclear resonant inelastic scattering of synchrotron radiation. We have found that the temperature-dependent change in the iron-specific phonon density of states of La 0.87 Ca 0.13 FePO was smaller than that of LaFeAsO 0.89 F 0.11 .

Crystal-Site-Selective Spectrum of Fe3O4 Obtained by Mössbauer Diffraction

We have succeeded for the first time in obtaining a crystal-site-selective Mossbauer spectrum of Fe3O4 using the Mossbauer diffractometer at SPring-8 BL11XU. In order to extract the nuclear resonant scattering, which provides the crystal-site-selective emission spectrum, reflections having a Bragg angle near 45° were used. The 666 and 10 10 0 reflection spectra reveal only B- and A-site spectra, respectively. Mossbauer diffraction spectroscopy enables us to measure the crystal-site-selective spectrum and to determine the precise hyperfine structure. This technique provides new possibilities for studying the local crystallographic and magnetic structure on iron sublattices in various multisite materials.

Development of time-domain interferometry for the study of glass formers

We developed time-domain interferometry (TDI) using 57Fe nuclear resonant scattering and applied to the study of the dynamics of ionic liquid 1-butyl-3-methylimidazolium iodide (BmimI). By using a multi-element avalanche photo diode as a detector, four TDI time spectra reflecting different momentum transfers were obtained at 250 K. By considering additional Gaussian broadening of resonant energy in stainless steel foils, the relaxation times of the density correlation in super-cooled BmimI were successfully obtained. The momentum transfer dependence of those relaxation times was confirmed to follow the Gaussian behaviour. Moreover, we discussed the trends of the TDI development in the future.

Spin Ordering in LaFeAsO and Its Suppression in F-doped Superconductors LaFeAsO1-xFx Probed by Mössbauer Spectroscopy

Mossbauer spectroscopy was applied to LaFeAsO and F-doped superconductors LaFeAsO 1- x F x ( x =0.07 and 0.11). The spectra of LaFeAsO clearly showed magnetic ordering below around 140 K, which corresponds to the structural phase transition temperature of around 150 K. On the other hand, the F-doped superconductors LaFeAsO 1- x F x showed no magnetic ordering down to 4.2 K. This fact clearly indicates the F doping suppresses the magnetic and structural phase transitions and allows them as superconductors.