Persistent Quest Research Activities 2002

6-2	Investigation of Local Crystal Structure Using gamma-Rays

Fig. 6-3

Structure of the fluorite-type dioxides, MO₂ (M=Ce, U, Th, Zr, Hf)

In the fluorite-type dioxides, the metal ion (M⁴⁺) is located in the center of a cube formed by 8 oxygen ions. The substitution of M⁴⁺ by a trivalent ion (e.g., Eu³⁺) creates oxygen vacancies. In deficient fluorite-type oxides, in which oxygen vacancies exist at random, the oxygen coordination numbers (CN) of Eu³⁺ and of the tetravalent ions (M⁴⁺) are thought to decrease from 8 to 6.

Fig. 6-4

Isomer shifts of the Eu³⁺ ion in the fluorite-type oxides
Eu_yM_1-yO_2-x (M=Ce, U, Th, Zr, Hf)
Different host metal ions (M⁴⁺) result in contrasting dependences of the Eu³⁺ isomer shift (IS) on the Eu composition y.
For the M=Ce and Th systems (

), the IS increases with y. Substitution of M⁴⁺ by Eu³⁺ causes the formation of oxygen vacancies (Eu_yM_1-yO_2-y/2) and a decrease in the CN of the Eu³⁺ ion from 8 (y=0) to 6 (y=1).
For the M=Zr and Hf systems (

), the IS has a minimum at y=0.5. This result indicates that an ordered pyrochlore phase is formed, that oxygen vacancies coordinate to the tetravalent ions (Zr⁴⁺, Hf⁴⁺), and that the CN of Eu³⁺ is 8.
For the M=U system (y<0.5), the IS remains almost constant, thus the CN of Eu³⁺ is considered to continue to remain at 8.

Mösbauer spectroscopy, recoilless nuclear resonance absorption of gamma-rays, is a powerful tool for probing the local electronic states around nuclei which absorb gamma-rays, by measuring the energy shift of the absorbed gamma-rays.
The local structure around Eu³⁺ in the fluorite-type oxides Eu_yM_1-yO_2-x (Fig. 6-3), (M=U, Th, Ce, Zr, Hf), was investigated by ¹⁵¹Eu Mösbauer spectroscopy over the whole range of ¹⁵¹Eu composition y.
Oxides Ln_yM_1-yO_2-x, in which tetravalent metal ions are substituted by trivalent lanthanide cations (Ln³⁺), are ionic conductors owing to the high mobility of the oxygen vacancies. The oxides with M=Ce, Zr, Hf, and Th have found application to oxygen sensors, fuel cells, etc. The material properties of the nuclear fuel oxides (M=U, Th) substituted by Ln, which is one of main fission products, are important for investigating the irradiation behavior of nuclear fuels.
In this work, we have found that the isomer shift of the Eu³⁺ (peak of the Mösbauer spectrum) depends on the coordination number of oxygen ions and on the Eu-O bond length.
Fig. 6-4 shows the isomer shift dependence on Eu composition y. This dependence reflects the different local structures in each system. For the M=Th, Ce systems, an oxygen-deficient fluorite- type solid solution Eu_yM_1-yO_2-x (x=y/2) was formed, and the coordination number of oxygen ions decreased from 8 to 6.
For the M=Zr, Hf systems, the isomer shift has minimum at y=0.5, and the system has an ordered pyrochlore structure in which the oxygen coordination number of Eu is 8 and that of Zr and Hf is 6. In the composition range 0.2<y<0.45, the Zr and Hf systems have fluorite-type structures from XRD measurements, but the isomer shift dependence shows an inverse trend for the Th and Ce systems. This result reveals that the Zr and Hf systems have a pyrochlore-type local structure in this composition range.
For the M=U system in the composition range 0<y<0.5, a fraction of the tetravalent U ions is oxidized to the pentavalent state in the substitution of U⁴⁺ by Eu³⁺. An oxygen-stoichiometric fluorite-type phase (Eu_y³⁺U_y⁵⁺U_1-2y⁴⁺O₂^2-) is found to be formed. Here, the oxygen coordination number is concluded to be 8, because there are no oxygen vacancies.

Reference
N.M. Masaki, et al., ¹⁵¹Eu Mösbauer Spectroscopic and XRD Study on Some Fluorite Solid Solution Systems, Eu_yM_1-yO_2-y/2 (M=Zr, Hf, Ce), Hyperfine Interactions (Proceedings of ICAME2001), (2002).

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Persistent Quest Research Activities 2002
Copyright(c) Japan Atomic Energy Research Institute

6-2

Investigation of Local Crystal Structure Using gamma-Rays