Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U) |
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Authors: | Nobuyuki M Masaki Haruyoshi Otobe Daijitsu Harada |
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Affiliation: | a Department of Materials Science, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan b Department of Chemistry, Hokkaido University, Sapporo 060-0810, Japan |
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Abstract: | 151Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the EuyM1−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-phase 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 Mössbauer 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 increasing 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, the lattice parameter, a0, of the system decreases almost linearly with y, in accordance with the calculated a0 versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu3+ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu2Zr2O7 and Eu2Hf2O7 (y = 0.5)) with O2−-eight-fold coordinated Eu3+(CN = 8). |
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Keywords: | A Oxides C Mö ssbauer spectroscopy C X-ray diffraction C Defects |
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