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

Abstract

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).