Masami Nakada

Iron oxidation state of a 2.45-Byr-old paleosol developed on mafic volcanics

The 2.45-Byr-old weathering profile developed on early Proterozoic mafic volcanics located near Cooper Lake, Ontario, Canada, was examined geochemically and mineralogically for a better understanding of the atmospheric oxygen evolution. Ferrous to ferric ion, Fe(II) and Fe(III), respectively, ratios of the bulk rock samples were analyzed by Mossbauer spectrometry. The total Fe (Fe(T)) and Fe(II) concentrations decrease from 12.0 and 11.2 wt.% to 1.85 and 0.89 wt.%, respectively, from the bottom to the top of the weathering profile. The Fe(T) and Fe(II) concentrations normalized to Ti and Zr, as well as the Fe(II)/Fe(III) ratio of raw data, linearly decrease with depth toward the top, while the Fe(III) concentration remains nearly constant throughout the profile. The linear decrease of Fe(II), accompanied by the nearly constant distribution of Fe(III), is difficult to be explained by the scenario of oxidizing weathering and subsequent reducing hydrothermal alteration. The behaviors of Fe(II) and Fe(III) can be simply explained by anoxic weathering. The anoxic weathering suggests that the 2.45-Ga atmosphere was anoxic. The slight increase of Fe/(Fe+Mg) in the octahedral sites of chlorite toward the top and no Ce anomaly in the REE patterns are also consistent with anoxic weathering.

NMR Evidence for the 8.5 K Phase Transition in Americium Dioxide

We report here the first NMR study of americium dioxide (AmO 2 ). More than 30 years ago, a phase transition was suggested to occur in this compound at 8.5 K based on magnetic susceptibility data, while no evidence had been obtained from microscopic measurements. We have prepared a powder sample of 243 AmO 2 containing 90 at. % 17 O and have performed 17 O NMR at temperatures ranging from 1.5 to 200 K. After a sudden drop of the 17 O NMR signal intensity below 8.5 K, at 1.5 K we have observed an extremely broad spectrum covering a range of ∼14 kOe in applied field. These data provide the first microscopic evidence for a phase transition as a bulk property in this system. In addition, the 17 O NMR spectrum has been found to split into two peaks in the paramagnetic state, an effect which has not been reported for actinide dioxides studied up to now. We suggest that the splitting is induced by self-radiation damage from the alpha decay of 243 Am.

Magnetic properties of some neptunyl(+1) complexes

A summary report on the magnetic properties of some neptunyl(+1) complexes (two formates; (NH4)[NpO2(O2CH)2](1), [NpO2(O2CH)(H2O)](2), and a phthalate, [(NpO2)2(O2C)2C6H4(H2O)3] (3)) is presented based on the magnetic and 237Np Mössbauer data obtained in our group for powder samples and the available single-crystal structural data. It is demonstrated that the remarkable correlations exist between the magnetic properties, the 237Np Mössbauer parameters and the structural features (dimensionality, neptunyl (O=Np=O) bonding network structure, etc.) among these uniquely low-dimensional neptunyl complexes. The magnetic data show that 1 with neptunyl (O=Np=O) 1D chain structure is paramagnetic (or only marginally ferromagnetic) down to 2 K, while 2 and 3 with similar 2D neptunyl sheet structure order ferromagnetically at Tc = 12 and 4.5 K, respectively. In addition, 3 having two kinds of Np sites exhibited a two-step magnetization behavior below Tc. Consistent with these magnetic data, 237Np Müssbauer spectra of 1 showed paramagnetic relaxation, while those of 2 and 3 showed a well resolved magnetic hyperfine splitting even above Tcs, indicating the existence of very slow paramagnetic relaxation. The all obeys the Curie-Weiss law in their paramagnetic temperature region up to 300 K. The values of the effective moments (and also of the saturation moments) of these systems are in the order, 1 > 2 > 3, and were smaller (and much smaller) even for 1 than those for free ion (Np5+(5f2)). These experimental findings on the present three neptunyl complexes are briefly discussed in connection with the reported magnetic properties of other neptunium compounds (pnictides, etc.) and also with a theoretical treatment based on an Ising-type behavior of individual neptunyl (O=Np=O)+ cations recently proposed by us.

238U Mössbauer study on the magnetic properties of uranium-based heavy fermion superconductors

We have performed 238U Mössbauer spectroscopy of uranium-based heavy fermion superconductors, UPd2Al3 and URu2Si2, in order to investigate their physical properties, mainly their magnetic properties. The slow relaxation of magnetic hyperfine interaction in a paramagnetic state and the static hyperfine field has been observed in an antiferromagnetic ordered state for each compound. The line-widths have maximum at their characteristic temperatures where their magnetic susceptibilities have maximum values.

Nuclear Magnetic Moment of the First Excited State (I= 2 +) of 238U

The 238 U Mossbauer effect of UO 2 has been investigated in the temperature range from 5.4 K to 280 K. Nuclear Zeeman splitting of the 238 U nucleus in antiferromagnetic UO 2 at 5.4 K is 59.1±3.9 mm s -1 and corresponds to the value of 252.3±0.5 T obtained by the 235 U pulsed NMR technique. The nuclear magnetic moment of the first excited state ( I =2 + ) of 238 U has been determined to be 0.254±0.015µ N . The conversion factor of 4.27±0.28 T/mm s -1 enables us to determine the hyperfine magnetic field at the 238 U nucleus using 238 U Mossbauer spectroscopy. The temperature dependence of the recoil-free fraction of the 238 U Mossbauer effect in UO 2 has been discussed.

Preparation of source and sealed absorber holder for237Np and238U Mössbauer measurements

Preparation of source and sample holder is described for the Mössbauer studies of237Np and238U. For the Mössbauer measurement of237Np, a source assembly with small sources of241Am metal was developed taking account of the transport regulations for radioactive substances. The source assembly of241Am showed a sufficient activity enough to measure the Mössbauer spectra of237Np. In order to handle237Np compounds safely, trebly sealed holders were designed which could encapsulate237Np samples without the seepage of liquid helium. A source for238U Mössbauer measurement was also developed from a highly pure242PuO2.

238U and 57Fe Mössbauer Spectroscopic Study of UFe2

We have performed 57Fe and 238U Mössbauer measurements of UFe2 in order to investigate its magnetic properties. From the results of 57Fe Mössbauer spectroscopy, the ferrromagnetic ordering at 167 K is caused by the iron 3d-electrons, which hybridize strongly with the uranium 5f-conduction electrons. It is also clarified from the results of 238U Mössbauer spectroscopy that there are no magnetic moments on the uranium atoms.

Synthesis of neptunyl(VI) hydroxides and their237Np Mössbauer spectra

Four types of neptunyl(VI) hydroxides have been synthesized by chemical oxidation of Np(IV) instead of ozone oxidation of Np(V) which caused the partial oxidation to the heptavalent state. NpO2(OH)2 (I) and NpO2(OH)2·H2O (orthorhombic type) (II) have been obtained by adding pyridine to the solution at 373K and 343K, respectively. NpO2(OH)2·H2O (hexagonal type) (III) and NpO2(OH)2·xH2O·yNH3 (x+y=1) (IV) have been prepared by using LiOH and NH4OH, respectively. The four materials have been characterized by X-ray powder diffraction patterns, thermogravimetric analysis and237Np Mössbauer spectra. The237Np Mössbauer spectrum of (I) measured first time as anhydrous neptunyl(VI) hydroxide (δ=−46.2 mm/s,e2qQ=193 mm/s and η=0.16 at 4.8K) has more distinct five-line Mössbauer pattern than those of (II), (III) and (IV). The Mössbauer spectra for (II), (III) and (IV) are slightly different from each other. The structural information has been obtained from these data.

Mössbauer spectroscopic study of frozen solutions of57Co using a specially designed cryostat for time differential coincidence emission Mössbauer spectroscopy of237Np

Experiments on the57Fe Mössbauer effect in frozen solutions were carried out in order to demonstrate the capabilities of a cryostat which was developed for the emission Mössbauer spectroscopy of237Np. We confirmed the existence of an aliovalent state of57Fe in the temperature range from 5.5 to 35 K. However, the relative amounts of Fe(II) and Fe(III) in the frozen matrix were almost constant for the temperature range from 5.5 to 150 K. The design, operation and capabilities of the cryostat are also described.

Emission Mössbauer spectroscopic studies on mixed-valence states of57Fe atoms produced in57Co-labeled trinuclear cobalt-iron carboxylates

Mixed-valence states of57Fe-atoms produced after EC-decay of57Co in oxo-centered trinuclear cobalt-iron carboxylates have been studied by means of emission Mössbauer spectroscopy. The emission Mössbauer spectra show nearly the same temperature dependence of the mixed-valence state as found in the absorption Mössbauer spectra of the corresponding trinuclear iron carboxylates. The results indicate that most of the decayed atoms keep their position at the sites of original57Co atoms.