Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Abdou, Maya; Gupta, Santosh K.; Zuniga, Jose P.; Mao, Yuanbing

doi:10.1016/j.jlumin.2019.02.059

Cited by 18 publications

(16 citation statements)

References 47 publications

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“…By increasing the U doping, a transformation between UO 6 6– (LHO:20%U) or UO 2 2+ (LHO:30%U) occurs in the LHO host which reduces oxygen vacancy concentration. In the GHO host, the UO 2 2+ type polymorph is stable at all investigated concentrations, driving the formation of a new defect phase at higher concentrations. , Moreover, Rietveld refinement confirms that the presence of O v in both host matrices distorts the octahedral coordination, which contributes to the calculated U 6+ –O bond length (2.30–2.33 Å) agreeing with the reported literature value (∼2.11 ± 0.22 Å). , This U 6+ –O phenomena is seen in other RE 2 Hf 2 O 7 compounds and is attributed to the structure of the host lattice, and their associated oxygen defect distribution, exhibiting U 6+ luminescent signatures . Based on these results, it is possible to control the U 6+ –O group structure in complex oxides by controlling the initial oxygen vacancy position, and this has important implications in the generation and storage of HLW without having to worry about adverse environmental impacts.…”

Section: Resultssupporting

confidence: 87%

“…However, when A 2 B 2 O 7 pyrochlore structures are exposed to radiation, two atomistic defects, cation antisite (0 → A B +B A ) and anion Frenkel pairs (0 →V O + O i ), are created . These defects are responsible for the pyrochlore to fluorite order–disorder phase transformation, which contributes to maintaining the crystallinity of the host material. , Among these compounds, RE-based hafnates RE 2 Hf 2 O 7 have been studied in the past decade as potential host materials for HLW due to their outstanding properties such as low thermal conductivity, high melting point, and chemical and thermal stability. , Of the two possible crystal structures (ordered pyrochlore and defect fluorite), the defect fluorite structure is more favorable when the ratio of the A and B cation radii is less than 1.46 with oxygen vacancies randomly distributed on the anion sites. On the other hand, the ordered pyrochlore structure is stable when this ratio is greater than 1.46 with an 8-fold oxygen coordinated RE 3+ site and a 6-fold oxygen coordinated Hf 4+ site. − While the ordered pyrochlore structure can have the dopant ions distributed in both cation sites, the defect fluorite structure has a better ability to tolerate radiation damage …”

Section: Introductionmentioning

confidence: 99%

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Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

et al. 2020

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Rare-earth based A 2 B 2 O 7 compounds have been considered as potential host materials for nuclear waste due to their exceptional chemical, physical, capability of accommodating high concentration of actinides at both A-and B-sites, negligible leaching, tendency to form antisite defects, and radiation stabilities. In this work, La 2 Hf 2 O 7 (LHO) and Gd 2 Hf 2 O 7 (GHO) nanoparticles (NPs) were chosen as the RE-based hafnates to study the structural changes and the formation of different U molecular structures upon doping (or alloying) at high concentration (up to 30 mol %) using a combined coprecipitation and molten-salt synthesis. These compounds form similar crystal structures, i.e., ordered pyrochlore (LHO) and disordered fluorite (GHO), but are expected to show different phase transformations at high U doping concentration. X-ray diffraction (XRD) and Rietveld refinement results show that the LHO:U NPs have high structural stability, whereas the GHO:U NPs exhibit a highly disordered structure at high U concentration. Alternatively, the vibrational spectra show an increasingly random oxygen distribution with U doping, driving the LHO:U NPs to the disordered fluorite phase. X-ray spectroscopy indicates that U is stabilized as different U 6+ species in both LHO and GHO hosts, resulting in the formation of oxygen vacancies stemming from the U local coordination and different phase transformation. Interestingly, the disordered fluorite phase has been reported to have increased radiation tolerance, suggesting multiple benefits associated with the LHO host. These results demonstrate the importance of the structural and chemical effect of actinide dopants on similar host matrices which are important for the development of REbased hafnates for nuclear waste hosts, sensors, thermal barrier coatings, and scintillator applications.

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Section: Resultssupporting

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

et al. 2020

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“…Principally, molten salt synthesis is a potential methodology that can lower the reaction temperature due to faster mass transport. In previous work (Mao et al, 2009;Gupta & Mao, 2021), La 2 Zr 2 O 7 pyrochlore ceramic was synthesized via the molten salt method by heating mixtures at 650 C for 6 h. However, until now, only limited work has been reported on actinide-incorporated pyrochlore via the molten salt method (Abdou et al, 2018(Abdou et al, , 2019Wang et al, 2020). Neodymium zirconate has also been proposed as a potential ceramic matrix for the immobilization of nuclear waste; therefore, in this work, we chose Nd 2 Zr 2 O 7 as a suitable host material for studying uranium immobilization via a molten salt process.…”

Section: Introductionmentioning

confidence: 99%

Controllable sites and high-capacity immobilization of uranium in Nd₂Zr₂O₇ pyrochlore

Sun

Zhou

et al. 2022

J Synchrotron Radiat

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As potential nuclear waste host matrices, two series of uranium-doped Nd2Zr2O7 nanoparticles were successfully synthesized using an optimized molten salt method in an air atmosphere. Our combined X-ray diffraction, Raman and X-ray absorption fine-structure (XAFS) spectroscopy studies reveal that uranium ions can precisely substitute the Nd site to form an Nd2–x U x Zr2O7+δ (0 ≤ x ≤ 0.2) system and the Zr site to form an Nd2Zr2–y U y O7+δ (0 ≤ y ≤ 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd 3 m) to defect fluorite (Fm 3 m) structures in both series of U-doped Nd2Zr2O7. The XAFS analysis indicates that uranium exists in the form of high-valent U6+ in all samples. To balance the extra charge for substituting Nd3+ or Zr4+ by U6+, additional oxygen is introduced accompanied by a large structural distortion; however, the Nd2Zr1.6U0.4O7+δ sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd2Zr2O7 host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd2Zr2O7 synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.

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“…[11,12] Uranium, as a critical radioactive element, has been widely used to produce nuclear power, which is increasing the necessity for uranium as an energy resource. [13][14][15][16] However, uranium and its radioactive isotopes contribute to a high-level of nuclear waste, which needs to be properly disposed. Therefore, we carried out speciation (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…oxidation state and coordination geometry) study of uranium ion doped in RE2Hf2O7 serves as a prerequisite for their capability for nuclear waste immobilization and eventually safe nuclear energy and sustainable environment. [15,16] Moreover, the terrestrial deposits of uranium are predicted to become a shortage owning to the booming energy demand. Seawater also contains uranium at low concentrations (~3.3 ppb).…”

Section: Introductionmentioning

confidence: 99%

Graphene Oxide-based Nanomaterials for Uranium Adsorptive Uptake

Liu¹,

Mao²

2021

ES Mater. Manuf.

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The growth of nuclear power generation and the necessity to acquire uranium reserves for energy security and pollution regulation for environmental protection put much emphasis on the removal and recovery of uranium from aqueous solutions. Adsorption has been proved to be a promising method for this purpose method because of its high adsorption efficiency, easy operation, low cost, reusability and availability of massive adsorbents. Among a wide variety of adsorbents, graphene oxide (GO) has demonstrated excellent adsorption potential for uranium uptake and recovery due to its unique 2D structure, high specific surface area and abundant oxygen-containing functional groups. Regarding the functional groups, it can make GO with high dispersion and hydrophilicity and participate in the complexation of uranium, leading to high adsorption efficiency for uranium. In this review, the research status and progress of GO-based nanomaterials for uranium adsorption are summarized. Their adsorption capacities, influencing factors, kinetics, isotherms and thermodynamics are compared and discussed. The microscopic mechanisms of uranium adsorption onto these GO-based nanomaterials are elaborated at molecular level by spectral analysis, surface complexation models, and theoretical calculations. Meanwhile, the challenges and research trends in the study of uranium adsorption by GO-based nanomaterials are pointed out. We believe that our focused review provides not only a summarizing reference on the current status of uranium removal and recovery by GObased nanomaterials, but also future directions for related follow-up research and practical applications.

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Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Cited by 18 publications

References 47 publications

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

Controllable sites and high-capacity immobilization of uranium in Nd₂Zr₂O₇ pyrochlore

Graphene Oxide-based Nanomaterials for Uranium Adsorptive Uptake

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Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Cited by 18 publications

References 47 publications

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE2Hf2O7 (RE = La and Gd) Nanoparticles

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE2Hf2O7 (RE = La and Gd) Nanoparticles

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore

Graphene Oxide-based Nanomaterials for Uranium Adsorptive Uptake

Contact Info

Product

Resources

About

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE₂Hf₂O₇ (RE = La and Gd) Nanoparticles

Controllable sites and high-capacity immobilization of uranium in Nd₂Zr₂O₇ pyrochlore