Structure and radiation response of anion excess bixbyite 
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Swift heavy ion irradiation was carried out to examine the ionization effects on structural changes of δ-Sc4Hf3O12 in which oxygen vacancies are regularly arranged. The specimens were irradiated at room temperature with 92 MeV xenon ions to fluences ranging from 3 × 1012 to 1 × 1014/cm2 and characterized by grazing (glancing) incidence x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. It was found that the pristine long-range ordered rhombohedral δ-phase undergoes a reconstructive transformation toward a long-range disordered cubic oxygen-deficient fluorite phase promoted by ionization effects. In addition, an ordered phase with a short-range structure different from the δ-type was formed in a layer going from the surface to a depth of ∼4.5 μm in the specimen irradiated to a fluence of 1 × 1014/cm2. It was found that the ordered phase is formed from the disordered cubic fluorite phase. This structural change is anomalous, because it is the opposite process of the usual irradiation-induced structural change, the order-to-disorder phase transformation. Electron diffraction experiments revealed that short-range ordered regions in this layer possess an oxygen-excess bixbyite organization (C-type heavy rare-earth oxides) with randomly filled anion vacant sites to account for the different stoichiometry and a long-range average oxygen-deficient fluorite phase.

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Anomalous structural phase transformation in swift heavy ion-irradiated δ-Sc4Hf3O12

Iwasaki

Kanazawa

Manago

et al. 2022

Journal of Applied Physics

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Microstructure evolution in 200-MeV Xe ion irradiated CeO2 doped with Gd2O3

Seo

Yasuda

Matsumura

et al. 2022

Journal of Applied Physics

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The microstructure of virgin and heavy ion-irradiated Ce1–xGdxO2–x/2 with a wide range of Gd dopant concentrations (0 ≤ xGd ≤ 0.5) was evaluated by x-ray diffraction (XRD), micro-Raman spectroscopy, and transmission electron microscopy (TEM) for selected area electron diffraction (SAED) analysis and plane-view bright-field (BF) imaging of ion tracks. The Ce1–xGdxO2–x/2 samples were irradiated with 200-MeV Xe14+ ions up to fluences from 3 × 1011 to 1 × 1013 cm−2 at ambient temperature. XRD patterns of the virgin Ce1–xGdxO2–x/2 samples showed saturation of lattice parameter and relaxation of microstrain in Ce1–xGdxO2–x/2 which are attributed to the increasing Gd concentration. Moreover, micro-Raman spectroscopy and SAED patterns revealed the bixbyite (C-type) structure formation and oxygen vacancy ordering for xGd > 0.2 that is induced by dispersed C-type domains in the fluorite-structured (F-type) matrix. In the irradiated samples, asymmetric XRD peaks induced by lattice distortion were observed together with ion tracks in BF-TEM images. The radiation damage was recovered with the increase of Gd concentration. There is a rapid reduction of radiation damage cross section for xGd > 0.2 with a reduction of the C-type structure reflections in XRD and SAED patterns.

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A review of the oxygen vacancy ordering in surrogate structures simulating Pu-based nuclear ceramics

Charlton¹,

Baldinozzi²,

Patel³

2023

Front. Nucl. Eng.

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Advanced nuclear power systems and nuclear fuel cycles will require nuclear fuels capable of higher burnup and with higher transuranic concentrations than those previously developed for current nuclear power plants. Expensive qualification tests are required to validate the thermal and mechanical performance of fuels in normal and accident-scenario operations. Research of surrogate systems with specific properties and characteristics of advanced nuclear fuels can be an effective way to frame the problem, reduce costs, and support the technical development of future research. From this perspective, lanthanide counterparts like mixed oxides of Ce and Nd can provide replica systems for many technological properties of the actual fuels. These ceramic systems can lead to a better understanding of the fundamental irradiation processes responsible for the evolution of their microstructures, the interplay with charge and defect localisation, and the evolution of their mechanical properties. In non-stoichiometric MO2−x binary systems (M = Ce, Pr, and Tb), there is evidence of systematic ordering of vacancies resulting in a deviation from the ideal fluorite structure and the formation of several intermediate fluorite-related phases. Substitution of the 4+ cations with 3+ cations in these systems drives the formation of oxygen vacancies as a charge compensation mechanism. By analogy with MO2−x systems, a variety of similar intermediate phases would also be expected to form in the MO2:Ln2O3 (Ln = La, Nd, Gd … etc). However, in order to achieve chemical homogeneity and charge ordering, prolonged annealing just above the charge ordering transition temperature is required, covering a time-scale determined by the chemical diffusion coefficient. Achieving these conditions with powder metallurgy techniques, commonly employed in literature, is practically impossible. This paper reviews the transport properties and structural features found in these surrogate systems which may be helpful in addressing challenges facing advanced nuclear fuels. We present results of a recent diffraction experiment investigating the structure of neodymium doped ceria synthesised using soft chemical methods. The sample shows a deviation from previous literature as the diffraction data is best described by a monoclinic Ln6O11-type structure (SG P21/c), often referred to as “β phase” in PrO2−x.

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Structure and radiation response of anion excess bixbyite Gd2Ce2O7

Cited by 3 publications

References 41 publications

Anomalous structural phase transformation in swift heavy ion-irradiated δ-Sc4Hf3O12

Anomalous structural phase transformation in swift heavy ion-irradiated δ-Sc4Hf3O12

Microstructure evolution in 200-MeV Xe ion irradiated CeO2 doped with Gd2O3

A review of the oxygen vacancy ordering in surrogate structures simulating Pu-based nuclear ceramics

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