151Eu-Mössbauer spectroscopic and X-ray diffraction study on Eu yM 1− yO 2− x (0 ≤ y ≤ 1.0) (M = Th, U)

Abstract

151Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu y M 1− y O 2− 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 ( V O) 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) Eu 2O 3). The Mössbauer results show that the isomer shifts (ISs) of Eu 3+ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O 2− around Eu 3+ with increasing y (CN = 8 − 2 y) ( x = y/2) results in the decrease of the average Eu O bond length, which is due to the decrease of repulsion force between O 2− anions. This result confirms that the IS of Eu 3+ correlates well with the average Eu O bond length in oxide systems. For the Eu–U system, the lattice parameter, a 0, of the system decreases almost linearly with y, in accordance with the calculated a 0 versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu 3+ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu 2Zr 2O 7 and Eu 2Hf 2O 7 ( y = 0.5)) with O 2−-eight-fold coordinated Eu 3+(CN = 8).