Phase variation and thermophysical properties of La2Hf2O7 with alumina addition

Gu, Shengyue; Zhang, Shouyang; Xue, Bei; Yan, Jinkai; Li, Wei; Zhang, Leilei

doi:10.1016/j.jeurceramsoc.2017.11.029

Cited by 15 publications

(8 citation statements)

References 28 publications

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“…Therefore, Raman spectroscopy is used to determine the local structure of the synthesized NPs because of its high sensitivity to the different M‐O vibrational modes (Figure B). Raman spectra from our NPs displayed six distinct peaks at 307, 322, 395, 493, 511, and 595 cm −1 corresponding to A 1g , E g , and 4F 2g modes which are typical of ordered pyrochlore structure . Specifically, the Raman vibrations at lower frequencies around 307, 322, 395 cm −1 due to F 2g , E g , and F 2g modes arise from vibrations of the La–O and Zr–O/Hf–O bonds whereas the higher frequencies of F 2g modes arise from stretching of the Zr–O/Hf–O bonds.…”

Section: Resultsmentioning

confidence: 84%

“…Raman spectra from our NPs displayed six distinct peaks at 307, 322, 395, 493, 511, and 595 cm −1 corresponding to A 1g , E g , and 4F 2g modes which are typical of ordered pyrochlore structure. 46 Specifically, the Raman vibrations at lower frequencies around 307, 322, 395 cm −1 due to F 2g , E g , and F 2g modes arise from vibrations of the La-O and Zr-O/ Hf-O bonds whereas the higher frequencies of F 2g modes arise from stretching of the Zr-O/Hf-O bonds. The Raman mode at 750 cm −1 in both hosts is attributed to ZrO 6 and HfO 6 distortions.…”

Section: Methodsmentioning

confidence: 96%

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On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

et al. 2019

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Unveiling the underlying mechanisms of properties of functional materials, including the luminescence differences among similar pyrochlores A2B2O7, opens new gateways to select proper hosts for various optoelectronic applications by scientists and engineers. For example, although La2Zr2O7 (LZO) and La2Hf2O7 (LHO) pyrochlores have similar chemical compositional and crystallographic structural features, they demonstrate different luminescence properties both before and after doped with Eu3+ ions. Based on our earlier work, LHO‐based nanophosphors display higher photo‐ and radioluminescence intensity, higher quantum efficiency, and longer excited state lifetime compared to LZO‐based nanophosphors. Moreover, under electronic O2−→Zr4+/Hf4+ transition excitation at 306 nm, undoped LHO nanoparticles (NPs) have only violet blue emission, whereas LZO NPs show violet blue and red emissions. In this study, we have combined experimental and density functional theory (DFT) based theoretical calculation to explain the observed results. First, we calculated the density of state (DOS) based on DFT and studied the energetics of ionized oxygen vacancies in the band gaps of LZO and LHO theoretically, which explain their underlying luminescence difference. For Eu3+‐doped NPs, we performed emission intensity and lifetime calculations and found that the LHOE NPs have higher host to dopant energy transfer efficiency than the LZOE NPs (59.3% vs 24.6%), which accounts for the optical performance superiority of the former over the latter. Moreover, by corroborating our experimental data with the DFT calculations, we suggest that the Eu3+ doping states in LHO present at exact energy position (both in majority and minority spin components) where oxygen defect states are located unlike those in LZO. Lastly, both the NPs show negligible photobleaching highlighting their potential for bioimaging applications. This current report provides a deeper understanding of the advantages of LHO over LZO as an advanced host for phosphors, scintillators, and fluoroimmunoassays.

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Section: Resultsmentioning

confidence: 84%

Section: Methodsmentioning

confidence: 96%

On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

et al. 2019

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“…The small size and narrow distribution of these NPs made by the MSS method are not achievable through conventional solid state and wet chemical reaction methods. For example, Gu et al , synthesized micron-sized pyrochlore La 2 Hf 2 O 7 powder at 550 °C using a combustion method, which would not have the efficacy of our NPs as thermally and chemically resistant support for catalysts and chemosensors …”

Section: Resultsmentioning

confidence: 99%

Pyrochlore Rare-Earth Hafnate RE₂Hf₂O₇ (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

et al. 2019

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Complex oxides of the RE2Hf2O7 series are functional materials that exist in the fluorite or pyrochlore phase depending on synthesis method and calcination temperature. In this study, we investigate the process of synthesis, crystal structure stabilization, and phase transition in a series of RE hafnate compounds, synthesized by the coprecipitation process of a single-source complex hydroxide precursor followed with direct calcination or molten-salt synthesis (MSS) method. Phase pure RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) ultrafine nanostructured powders were obtained after calcinating the precursor in a molten salt at 650 °C for 6 h. Moreover, we demonstrate that the MSS method can successfully stabilize ideal pyrochlore structures for La2Hf2O7 and Pr2Hf2O7 in the nanodomain, which is not possible to achieve by direct calcination of the coprecipitated precursor at 650 °C. We propose mechanisms to elucidate the differences in these two synthesis methods and highlight the superiority of the MSS method for the production of RE hafnate nanoparticles.

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“…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%

“…11 These defects are responsible for the pyrochlore to fluorite order− disorder phase transformation, which contributes to maintaining the crystallinity of the host material. 11,12 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 13 due to their outstanding properties such as low thermal conductivity, 14 high melting point, 15 and chemical and thermal stability. 16,17 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.…”

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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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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Phase variation and thermophysical properties of La2Hf2O7 with alumina addition

Cited by 15 publications

References 28 publications

On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

Pyrochlore Rare-Earth Hafnate RE₂Hf₂O₇ (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

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

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Phase variation and thermophysical properties of La2Hf2O7 with alumina addition

Cited by 15 publications

References 28 publications

On comparison of luminescence properties of La2Zr2O7 and La2Hf2O7 nanoparticles

On comparison of luminescence properties of La2Zr2O7 and La2Hf2O7 nanoparticles

Pyrochlore Rare-Earth Hafnate RE2Hf2O7 (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

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

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On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

On comparison of luminescence properties of La₂Zr₂O₇ and La₂Hf₂O₇ nanoparticles

Pyrochlore Rare-Earth Hafnate RE₂Hf₂O₇ (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

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