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Abstract  

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.

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Abstract  

Coincidence Mössbauer spectra of57Co-labelled [CoFe2O(CH3CO2)6(H2O)3] were determined at 78 K and 298 K with three timewindows of 0–50, 50–150 and 150–300 ns. Temperature dependence in the spectral shape ascribed to an intramolecular electron transfer was observed in all the time-window spectra, while little time dependence was observed. The results indicate that57Fe atoms produced by EC-decay are incorporated in a chemical environment similar to that of the parent57Co atoms, forming a trinuclear FeIIFe2 III structure at an early stage after the EC-decay.

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Abstract  

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.

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Abstract  

A pentavalent neptunium glycolate complex, [NpO2(O2CCH2OH)(H2O)], was studied by237Np Mössbauer spectroscopy. The Mössbauer spectra were strongly influenced by paramagnetic relaxation at 5.3, 20, and 42 K. Least-squares fitting of the spectra accompanied with relaxation effect was attempted by assuming a pure Zeeman type paramagnetic hyperfine interaction. The estimated isomer shift value falls within a range of the previously reported values for seven-coordinated Np(V) compounds.

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Abstract  

KX-ray-gated emission Mössbauer spectra of57Co-labelled CoF2 and CoF2.2H2O were measured at room temperature, using the coincidence technique. A difference was found in the relative intensity of57Fe(II)/57Fe(III) between the X-ray-gated and non-gated emission spectra. The results are explained in terms of local radiolytic effects of water of crystallization and the chemical effects associated with the de-excitation processes caused by EC-decay.

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Summary  

The correlations of isomer shifts in various neptunyl(V and VI) compounds with crystallographic structures were revaluated. A linear correlation between the isomer shifts of neptunyl(VI) compounds and Np-O bond lengths of neptunyl group has been demonstrated. On the other hand, it has been evidenced that the isomer shifts of neptunyl(V) compounds are correlated much stronger with the mean Np-O distances in the crystals than the lengths in neptunyl(V) group.

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Abstract  

Five neptunyl(V) compounds were synthesized and studied by237Np Mössbauer spectroscopy. The isomer shifts (δ) of the Mössbauer spectra ranged from −18.6 to −19.1 mm/s for the compounds with Np atoms surrounded by 7 oxygen atoms (coordination number (CN) 7). On the other hand, the larger value of δ was obtained for the compound with CN 8. from the comparison of the present results with those reported on neptunyl(V) and (VI) compounds, it is concluded that there is a correlation between the δ and the CN for neptunyl(V) compounds, and the distribution of δ is narrower for neptunyl(V) compounds than that of neptunyl(VI) compounds.

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Abstract  

237Np Mössbauer spectrum of the heptavalent neptunium compound [Co(NH3)6][NpO4(OH)2]·2H2O was remeasured for certification of the previous investigations. The isomer shift value of the Mössbauer spectrum showed a characteristic value for the heptavalent neptunium at 4.7K. Although the spectrum seemed to consist of two quadrupole-split peaks as previously reported byStone et al., the fitting with using two quadrupole-split peaks did not give a good agreement withStone's data.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: T. Saito, J. Wang, T. Kitazawa, M. Takahashi, M. Takeda, M. Nakada, T. Nakamoto, N. Masaki, T. Yamashita, and M. Saeki

Abstract  

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·xH2yNH3 (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,e 2 qQ=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.

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