Haruyoshi Otobe

Correlation of crystal structures with electric field gradients in the fluorite- and pyrochlore-type compounds in the Gd2O3–ZrO2 system

Abstract Correlation of crystal structure with electric field gradient (EFG) in the fluorite- and pyrochlore-type compounds in the Gd2O3–ZrO2 system GdxZr1−xO2−x/2 with 0.18⩽x⩽0.62 were investigated by 155Gd Mossbauer spectroscopy, powder X-ray diffraction and point-charge model (PCM) calculation. An intermediate ordered pyrochlore phase forms for 0.45⩽x⩽0.55, sandwiched with a disordered fluorite phase for 0.18⩽x

Oxygen potential measurements of pyrochlore-type Zr2M2O7+x (M= Pu, Ce) by EMF method

Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia oxygen sensor, and investigated the relation between oxygen potential (g(O2)), oxygen composition (x) and temperature (T) for oxygen-excess pyrochlore Zr2Pu2O7+x between 798 K and 1078 K over 0.42<x<0.95. Similar experiments were also carried out for P-type Zr2Ce2O7+x. It was found that g(O2) of P-type Zr2Pu2O7+x. are significantly higher than those of oxygen deficient fluorite PuO2-x at the same values of the oxygen to metal molar ratio (O/M) or the molar ratio of Pu(Ce)3+ ions to the total Pu(Ce)3++Pu(Ce)4+ions (y). g(O2) of P-type Zr2Pu2O7+x. are about -150 kJ/mol lower than those of P-type Zr2Ce2O7+x at the same values of O/M or y, though the shape of g(O2) versus x curves resembles each other over the range of x investigated in this study. These results are consistent with the fact that g(O2) of PuO2-x are about -150 kJ/mol lower than those of CeO2-x at the same values of O/M or y, and their g(O2) curves resemble each other.

151Eu Mössbauer Spectroscopic Study on the EuyM1-yO2-x (0 ≤ y ≤ 1.0) (M=Th, U)

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).

151Eu Mössbauer spectroscopic and XRD study on some fluorite-type solid solution systems, EuyM1−yO2−y/2 (M=Zr, Hf, Ce)

151Eu 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).