¹⁵¹Eu Moessbauer spectroscopic study on the Eu_yM_1-yO_2-x (0 ≤ y ≤ 1.0) (M=Th, U)

¹⁵¹Eu-Moessbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1-yO_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-phases regions sandwiching a narrow C-type (C) single phase around y approx. = 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y <1.0 (C + B-type (monoclinic) Eu₂O₃). The Moessbauer 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 O^2- 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 O^2- 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). Consistently 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 O^2--8 fold coordinated Eu³⁺ (CN=8). (author)