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On Structure and Phase Transformation of Uranium Doped La2hf2o7 Nanoparticles As an Cite This: Mater Volume 2 | Number 12 | December 2018 MATERIALS CHEMISTRY FRONTIERS rsc.li/frontiers-materials MATERIALS CHEMISTRY FRONTIERS View Article Online RESEARCH ARTICLE View Journal | View Issue On structure and phase transformation of uranium doped La2Hf2O7 nanoparticles as an Cite this: Mater. Chem. Front., 2018, 2, 2201 efficient nuclear waste host† Maya Abdou,a Santosh K. Gupta, ab Jose P. Zunigaa and Yuanbing Mao *ac The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La2Hf2O7 nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition. 4+ We have discovered that in our La2Hf2O7:U NPs, the uranium dopants are stabilized at both U and 6+ 6+ 6À U oxidation states in which the U oxidation state exists in octahedral uranate UO6 form. We also 4+ 4+ 6À confirmed that the U ions substituted the Hf ions with a lifetime of B1.0 ms and the UO6 ions resided at the La3+ sites with a lifetime of B9.0 ms. More interestingly, the proportion of the U4+ ions in 6À the La2Hf2O7:U NPs was higher than that of the UO6 ions at low doping level, but at the doping level 6À 4+ higher than 2.5%, the fraction of the UO6 ions was greater than that of the U ions. Furthermore, we studied the structural phase transformation from order pyrochlore to cotunnite of these La2Hf2O7:U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U4+ ions 6À Received 30th May 2018, whereas disordered cotunnite phase favors the UO6 ions. We further used in situ Raman spectroscopy Accepted 22nd June 2018 to confirm the reversible cotunnite to pyrochlore phase transformation of the La2Hf2O7:10%U NPs at DOI: 10.1039/c8qm00266e 900 1C. Therefore, this work demonstrated the successful development of uranium doped La2Hf2O7 NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase rsc.li/frontiers-materials transformation, and structure–optical property correlation. Published on 26 June 2018. Downloaded 6/15/2019 7:49:41 PM. 1. Introduction Uranium ion has multiple oxidation states (i.e. +3, +4, +5 and +6), all of which are luminescence active with characteristics 16–19 Compounds with a general formula of A2B2O7 have received emission. Speciation studies of uranium ion in A2B2O7 intense attention due to their high thermodynamic stability, compounds will be highly beneficial for fundamental uranium high radiation stability, capability to incorporate lanthanides chemistry and nuclear industry. However, because of its com- and actinides, and ability to form antisite defects by swapping A plex nature and various existing valence states, the incorpora- and B positions.1–6 Among these compounds, rare-earth hafnates tion mechanism, oxidation state and structural environment of RE2Hf2O7 possess various desirable properties, which are very uranium ions in A2B2O7 compounds are uncharted and vague. important for different technological applications, such as com- For example, Zhang and coworkers investigated the phase 7 8 20 puter tomography (CT), positron emission tomography (PET), evolution of U doped Y2Ti2O7 and Gd2Ti2O7. They have carried 9 10 high-energy radiation detectors, scintillation host materials, out detailed phase evolution investigation from Ln2Ti2O7 pyro- 11 4,9,12–15 magnetic materials, among others. chlores to Ln0.5U0.5Ti2O6 (Ln = Y and Gd) brannerites in glasses Uranium and its radioactive isotopes contribute to a high- using various techniques such as X-ray diffraction, Raman level of nuclear waste, which needs to be properly disposed. spectroscopy, diffuse reflectance spectroscopy and electron microscopy. Shu et al. studied the effect of alpha irradiation 21 a Department of Chemistry, University of Texas Rio Grande Valley, on U doped Gd2Zr2O7. Their study revealed that main crystal 1201 West University Drive, Edinburg, Texas 78539, USA. structure does not change but weak structural ordering takes E-mail: [email protected]; Tel: +1-956-665-2986 place on alpha irradiation. They have also found increase in b Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, radiation stability of Gd2Zr2O7 at high uranium concentrations. Mumbai-400085, India c School of Earth, Environmental, and Marine Sciences, University of Texas Rio Lu et al. explored the effects of U3O8 on the phase and Grande Valley, 1201 West University Drive, Edinburg, Texas 78539, USA microstructure evolution of Gd2Zr2O7 and found that uranium † Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00266e is homogenously distributed in +4 and +6 oxidation states at This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2018 Mater. Chem. Front., 2018, 2, 2201--2211 | 2201 View Article Online Research Article Materials Chemistry Frontiers Gd3+ and Zr4+ sites, respectively.22 One of us studied the specia- identify the phase and rule out the formation of any additional tion of uranium using luminescence spectroscopy for A2Zr2O7 phase of the La2Hf2O7:x%U NPs. Morphostructural character- (A = La, Nd and Gd) and the results show that uranium is ization of the synthesized NPs was performed with scanning 4+ 6À 6À stabilized as U and UO6 in Gd2Zr2O7,onlyasUO6 in electron microscopy (SEM) and high-resolution transmission 6À 2+ 23–25 Nd2Zr2O7, and as both UO6 and UO2 in La2Zr2O7. There electron microscopy (HRTEM) as shown in Fig. S4 and S5 (ESI†). are few more reports on uranium doped Gd2Zr2O7 wherein The presence of uranium dopant was confirmed by energy dis- speciation of uranium was carried out in bulk phase using persive spectroscopy (EDS) (Fig. S6, ESI†). The doping efficiency X-ray diffraction, Raman spectroscopy and X-ray photoelectron and uniform distribution of constituent element were investi- spectroscopy along with other suitable techniques.26–28 However, gated using elemental mapping (Fig. S7, ESI†). none of these reports includes studies on structure and phase 3.2 Raman spectroscopy evolution of uranium doped La2Hf2O7 nanoparticles (NPs). Photoluminescence (PL) spectroscopy is an indispensable It is well known that the ideal pyrochlore phase has six technique to probe optical properties and local structure of well-resolved Raman active vibrational modes in the range of À1 phosphors. It is also the most acceptable technique for detecting 200–1000 cm which are represented as GP =A1g +Eg +4F2g, and estimating uranium in ultra-trace level in both solids as well whereas the fluorite phase has mainly one Raman active mode 29 32 as aqueous media. Meanwhile, Raman spectroscopy has been that is GF =F2g. This is because seven oxygen ions are widely utilized as a tool to distinguish between the disordered randomly distributed at eight anion positions, which leads to fluorite and the ordered pyrochlore phases of A2B2O7 com- structural disordering in the fluorite phase, and hence all the pounds. Due to its high sensitivity to oxygen-cation vibrations, fine peaks of pyrochlore collapsed into one broader peak. More 0 Raman spectroscopy has the capability to probe local structure specifically, phase transformation from A2B2O6O pyrochlore % % and to further identify disorder within the pyrochlore structure, (Fd3m space group) to AO2 fluorite (Fm3m, Z = 4) structure takes which emerge from vacancies and defects that disrupt the place through the disappearance of A1g and Eg Raman modes translational symmetry. Moreover, X-ray photoelectron spectro- and decrease in the number of F2g modes from 4 to 1. scopy (XPS) is a surface-sensitive quantitative spectroscopic Radius ratio (rA/rB) plays an important role in determining 33 technique that measures the elemental composition, empirical which structure of A2B2O7 compounds attain. It has been formula, chemical state and electronic state of the elements that reported that fluorite phase is more likely to form if rA/rB o 1.46 exist within a material. It is also one of the highly sensitive and while ordered pyrochlore phase is more likely to be methods to probe oxygen vacancies. Therefore, in this work, we stabilized if rA/rB is greater than 1.46 at room temperature. have used these three techniques together along with other It was propose that rA/rB for different A2B2O7 compositions traditional materials characterization techniques complemen- follows this trend: disordered fluorite phase (DFP) rA/rB o 1.21 o tarily to investigate the structure and phase transition of the d-phase rA/rB o 1.42–1.44 o ordered pyrochlore phase (OPP) 34 La2Hf2O7 NPs with different uranium doping levels. Lastly, for rA/rB o 1.78–1.83 o monoclinic pyrochlore rA/rB o 1.92. the first time, we observed reversible phase transformation La2Hf2O7 is the most favorable candidate with radius ratio of Published on 26 June 2018. Downloaded 6/15/2019 7:49:41 PM. in the La2Hf2O7:10%U NPs using in situ Raman measurement. 1.45 to be stabilized in pyrochlore phase. Fig. 1a shows the Therefore, through the successful development of uranium Raman spectra of the as-synthesized La2Hf2O7:U NPs with differ- doped La2Hf2O7 NPs and their thorough characterization of ent uranium concentrations. Undoped sample has six well- the fundamental spectra of uranium ions, doping induced phase resolved Raman peaks at 306, 324, 402, 501, 521 and 601 cmÀ1 transformation, and structure–optical property correlation, we pertaining to the vibrations of La–O and Hf–O bonds. These believe this work open new research areas important for safe peaks are assigned to F2g,Eg,F2g,A1g F2g and F2g modes, 35 nuclear energy and sustainable environment. respectively. The vibrational modes of F2g,Eg, and F2g at low frequency region of 300–400 cmÀ1 arise from vibrations of the La–O and Hf–O bonds. On the other hand, the high frequency 2.
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