We applied Mössbauer spectroscopy to 237Np in nitric acid solutions to study chemical states. Sample solutions were adjusted to 50 mg/ml of Np(V) and Np(VI) concentration in 0.6 M nitric acid solutions. These solutions were put into original designed containers for samples and mounted to sample holders. The values of isomer shift (IS, relative to NpAl2) of Np(V) and Np(VI) in 0.6 M nitric acid were −17.4(1) and −37.2(3) mm/s at about 10 K, respectively. It was found that Np(V) had 7 coordination number and Np(VI) had 8 in nitric acid solutions from values of IS.

151Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the EuyU1-yO2-x (M=Th, and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen-vacancies (Vo) are disordered (x=y/2) is formed for 0 ≤ y ≤ 0.5 and that two-phases regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu2O3). The Mossbauer results show that the isomer shifts (ISs) of Eu3+ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O2− around Eu3+ with y (CN=8-2y)(x=y/2) results in the decrease of the average Eu-O bond length, which is due to the decrease of repulsion force between O2- anions. This result confirms that the IS of Eu3+ correlates well with the average Eu-O bond length in oxide systems. For the Eu-U system, also the defect-fluorite (DF)-type solid solution phase is formed for y < 0.5, but the lattice parameter (a0) of the system decreases almost linearly with y, in accordance with the calculated a0 vs. y curve for the oxygen-stoichiometric (i.e. x=0) fluorite dioxide (CN=8). Consistent with this XRD result, the ISs of Eu3+ in this composition range are smaller than those of the Eu-Th system and remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu2Zr2O7 and Eu2Hf2O7 (y=0.5)) with O2--8 fold coordinated Eu3+(CN=8).

Eu-151 Mossbauer spectroscopy and powder x-ray diffractometry (XRD) have been applied to elucidate the local structure around the trivalent europium (Eu3+) in the titled three defect-fluorite systems. From this study, it has been clarified that in contrast to Eu-Ce system the stabilized cubic-fluorite phase in Eu-Zr and Eu-Hf systems has most likely a pyrochlore-type local structure (microdomain structure).

A nuclear magnetic resonance (NMR) study is reported for a 5f-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, UO2, where we have performed magnetic actinide (U-235) as well as nonmagnetic ligand (O-17) NMR studies in a SS-electron system. The observed U-235 hyperfine interaction is consistent with an axially symmetric Sf-wave-function character for U4+ below a first-order transition temperature T-N = 30.8 K Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the O-17 spin-echo decay provide evidence of a lattice distortion just below T-N, which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between 5f quadrupoles and lattice distortion, the behavior of the 5f quadrupoles can be investigated using uranium NMR, white the lattice distortion can be studied with the nonmagnetic Ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear 3k-type structure is the most likely, because either the 1k or 2k would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates T-1(-1) at both sites shaw a T-7 behavior below T-N, which suggests the presence of gapless excitations due to strong magnon-phonon coupling.

We have measured the Np-237 Mossbauer spectra of NpO2 whose specimens have been stored for 12 years before and after annealing at 773, 973, 1373 K, The Mossbauer spectrum of NpO2 after annealing consists of a symmetrical absorption line, while the line shape of the sample before annealing is broad and asymmetric below 25 K. The change of line shape is attributed to the phase transition at 25 K which might be enhanced by the existence of the self-irradiation-induced defects. Irradiation-induced damage can be recovered by annealing above 973 K. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

A typical hyperfine structure of U4+ ions is obtained by the U-235 NMR in the antiferromagnetic state of UO2. The nuclear electric quadrupole interaction indicates that the Sf-wave function in the cubic crystalline field is reduced to the axial symmetry below T-N. The nuclear spin-lattice relaxation rate at both uranium and oxygen sites shows a T-7-like behavior in the antiferromagnetic state, which indicates the presence of a gapless spin-wave excitation. (C) 2000 Elsevier Science B.V. All rights reserved.

Five neptunyl(V) compounds were synthesized and studied by 237 Np 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.

A series of double-perovskite oxides Ba2LnNbO6 (Ln=lanthanide elements) were synthesized. Their powder X-ray diffraction measurements and Rietveld analysis show that they are monoclinic with space group P21/n and that Ln3+ and Nb5+ ions are structurally ordered. Although the average Ln–O bond length increases with the ionic radius of Ln3+, the average Nb–O bond length is nearly constant for all the Ba2LnNbO6. Magnetic susceptibility measurements show that they are paramagnetic down to 2 K. The 151Eu Mössbauer spectrum for Ba2EuNbO6 indicates that the Eu ion is in the trivalent state and a quadrupole interaction exists in this compound. Both the magnetic susceptibility and the electron paramagnetic resonance (EPR) spectrum measurements show that the Gd3+ ion is in the 8S7/2 state. In addition, the anisotropic EPR spectrum indicates that the six oxygen coordination around the Gd3+ ion has no longer a perfect octahedral symmetry.

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.

237 Np Mössbauer spectrum of the heptavalent neptunium compound [Co(NH 3 ) 6 ][NpO 4 (OH) 2 ]·2H 2 O 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 by Stone et al., the fitting with using two quadrupole-split peaks did not give a good agreement with Stone 's data.

Solid solutions Sr2−xLaxFeO4−δ (0≤x≤0.5) with the K2NiF4-type structure have been prepared, and their magnetic susceptibilities have been measured in the temperature range between 4.2 and 300 K. Substitution of the La ion for the Sr site considerably weakens the antiferromagnetic transition of Sr2FeO4, and it finally disappears in Sr1.6La0.4FeO4−δ. Below the magnetic transition temperatures, magnetic susceptibilities measured under zero-field cooled and field cooled conditions indicate the formation of a spin-glass state. Mössbauer spectra for Sr1.9La0.1FeO4−δ at room temperature and at 9 K have been measured, and the spectra at 9 K show the presence of magnetic hyperfine interaction of Fe4+ ions; there is no charge disproportionation of Fe4+→Fe3+ + Fe5+, even below the magnetic transition temperature.

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.

The U-238 Mossbauer effect of UO2 has been investigated in the temperature range from 5.4 K to 280 K. Nuclear Zeeman splitting of the U-238 nucleus in antiferromagnetic UO2 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 U-235 pulsed NMR technique. The nuclear magnetic moment of the first excited state (I = 2(+)) of U-238 has been determined to be 0.254 +/- 0.0151 mu(N). The conversion factor of 4.27 +/- 0.28 T/mm s(-1) enables us to U-238 Mossbauer spectroscopy. determine the hyperfine magnetic field at the U-238 nucleus using The temperature dependence of the recoil-free fraction of the U-238 Mossbauer effect in UO2 has been discussed.

A pentavalent neptunium glycolate complex, [NpO 2 (O 2 CCH 2 OH)(H 2 O)], was studied by 237 Np 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.

Five neptunyl(V) compounds were synthesized and studied by 237Np Mössbauer spectroscopy. The isomer shifts (δ) of the Mössbauer spectra ranged from −18.6 to −19.0 mms−1 for the compounds with Np atoms surrounded by seven oxygen atoms. Interlaced two sets of hyperfine splitting were observed in the spectra for the acetate and the benzoate, δ's of one set with larger magnetic splitting were −19.4 and −19.5 mms−1, respectively, but those of the other set with smaller magnetic splitting were −13.2 and −10.8 mms−1, respectively. From the comparison of the present results with those reported on neptunyl(V) and (VI) compounds, it is concluded that there are two neptunium sites in the acetate and the benzoate in their structure, one is the site with the coordination number (CN) of 7 and the other is that with CN of 8.

The magnetic properties of the layered perovskite compounds NaLnTiO4(Ln=Sm, Eu, and Gd) are reported. Their dc susceptibilities were measured from 4.5 to 320 K. The Mössbauer spectrum of151Eu and the electron paramagnetic resonance (EPR) spectrum of Gd3+were also measured at room temperature. NaEuTiO4shows Van Vleck paramagnetism which corresponds to the singlet ground state7F0of Eu3+ions, with a spin–orbit coupling constantγ=323 cm−1. NaSmTiO4also behaves as a Van Vleck paramagnet. The magnetic susceptiblity of NaGdTiO4follows the Curie law. The Mössbauer spectrum of NaEuTiO4clearly shows that Eu is in the trivalent state, and a quadrupole interaction is found to exist in this compound. From the EPR measurement thegvalue of Gd3+in NaGdTiO4is determined to be 1.99. Both the magnetic susceptibility results and the EPR measurements show that Gd3+ion in this compound is scarcely affected by the crystal field.

Preparation of source and sample holder is described for the Mössbauer studies of 237 Np and 238 U. For the Mössbauer measurement of 237 Np, a source assembly with small sources of 241 Am metal was developed taking account of the transport regulations for radioactive substances. The source assembly of 241 Am showed a sufficient activity enough to measure the Mössbauer spectra of 237 Np. In order to handle 237 Np compounds safely, trebly sealed holders were designed which could encapsulate 237 Np samples without the seepage of liquid helium. A source for 238 U Mössbauer measurement was also developed from a highly pure 242 PuO 2 .

Hydrogen molecules (H2) and atoms (H) formed by 60Co γ-ray irradiation of type Y zeolites adsorbing various amounts of water were measured by means of gas chromatography and ESR spectroscopy. The enhancement of H2 formation due to energy transfer from the zeolites to adsorbed water was observed. The discussion of H and H2 formation by energy transfer was made based on the comparison of G(H2) and G(H) between NaY- and HY-water system. The dependence of G(H2) upon preheating temperature of zeolites implied that both Brönsted acidic nature of hydroxyls and Lewis sites formed during dehydroxylation influence the H2 and H yield in the HY-water system.