151Eu-Mössbauer spectroscopic and X-ray diffraction study on 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 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 EuO 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 EuO 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).