Disordered Fluorite Structure Research Articles

Growth of ultimate high-density scintillating films for high-resolution X-ray imaging at synchrotrons

Scintillators are X-ray to visible light converters applied in X-ray imaging detectors used at synchrotrons. Drastically improved performances of the 4th generation synchrotron sources enable us to perform X-ray imaging experiments at higher energies. For high spatial resolution X-ray imaging (micrometer to submicrometer), thin scintillating films are required. Consequently, especially for high X-ray energies (30keV to 100keV), the detection efficiency is limited due to the low X-ray absorption efficiency of the thin films. We have used Liquid Phase Epitaxy (LPE) to grow thin films of one of the ultimate high-density materials, Lu2Hf2O7:Eu3+, which demonstrate scintillating properties and thus combine high spatial resolution and maximized absorption efficiency. X-ray imaging experiments demonstrate that the Modulation Transfer Function (MTF) reaches 10% at 900 lp/mm, and radiographs visually confirm the promising imaging properties. We present structural, luminescent, and scintillation characterization of Lu2Hf2O7:Eu thin films grown on ZrO2:Y substrates, showing that the films crystallize in the disordered fluorite structure and the europium ions incorporate into the structure and enhance the luminescence intensity. This contribution is a first step toward developing promising ultra-dense, hafnate-based scintillating screens for high spatial resolution X-ray imaging.

Plasma sprayed Yb4Hf3O12 thermal barrier coatings with excellent thermophysical properties and robust CMAS corrosion resistance

Here we reported atmospheric plasma sprayed Yb4Hf3O12 as a novel thermal barrier coating (TBC) and its comprehensive properties including phase composition, high-temperature stability, thermal expansion coefficient (TEC) and thermal conductivity were characterized. XRD and Raman results co-verify as-sprayed Yb4Hf3O12 coating is composed of disordered fluorite structure, and would return to its original δ structure during 1300 °C exposure. The linear relationship between thermal expansion rate and temperature indicates the fluorite→δ phase transformation accompanies no significant volumetric effect and thus might not pose detriment to coating durability. The Yb4Hf3O12 coating also combines many attractive characteristics, from high TEC (near 10.3 × 10−6 K−1, 200-1500 °C) and thermal insulation capability (0.95–1.12 W/m·K, room temperature-1000 °C) to thermal stability, and sintering resistance. We also comparatively studied the infiltration behaviour of natural volcanic ash and synthetic sandy CMAS within coating at 1250 °C. Robust corrosion resistance of Yb4Hf3O12 coating are demonstrated, especially against volcanic ash. These preliminary results suggest that Yb4Hf3O12 might be a lone ceramic topcoat and capable of operating well beyond the limits of traditional yttria stabilized zirconia (YSZ).

Helium irradiation effects on MgO-Nd2Zr2O7 composite ceramics prepared by one-step method used for inert matrix fuel

In this work, the helium irradiation effects were systematically investigated on MgO-Nd2Zr2O7 (hereafter M-NZO) composite ceramics used for inert matrix fuel, in which the ion fluences range from 5 × 1016 to 3 × 1017 ions/cm2 at room temperature. It was demonstrated that M-NZO composite ceramic shows superior radiation tolerance after He implantation. Specifically, MgO had almost no phase transition and extensive amorphization. And Nd2Zr2O7 phase possessed slightly pyrochlore-fluorite-amorphization structural transformation, while part of Nd2Zr2O7 maintained the ordered pyrochlore structure and co-existed with the irradiation-induced disordered fluorite structure. Importantly, GIXRD results indicated that MgO phase presents higher resistance to radiated damage than Nd2Zr2O7 phase. Besides, SEM images illustrated that MgO particles have substantially less fragmentation than Nd2Zr2O7 particles due to the higher thermal conductivity of MgO. It reconfirmed that MgO phase resists irradiation damage better than Nd2Zr2O7 phase in post-irradiated M-NZO composite ceramics.

Preparation and irradiation stability of A2B2O7 pyrochlore high-entropy ceramic for immobilization of high-level nuclear waste

The high-entropy pyrochlores have attracted increasing attention due to their ability to immobilize multiple nuclides simultaneously, compared with baseline single-component pyrochlores. In this work, nine kinds of multicomponent oxide ceramics are synthesized and eight of them exhibit a single pyrochlore phase. In addition, the ratio of rA to rB of high-entropy pyrochlore Gd2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 is 1.4499, this one observation suggests that for high-entropy pyrochlore, the rA/rB limit of pyrochlore phase is different from that of single-component pyrochlore (1.46 < rA/rB < 1.78). In order to evaluate the radiation resistance of high-entropy pyrochlores, four typical samples were irradiated by 800 keV Kr2+ ions beam. The GIXRD results show that all the high-entropy pyrochlores only undergo a phase transition from an ordered pyrochlore structure to a disordered fluorite structure (O-D transition) at the highest irradiation dose (∼ 40 dpa) in this study, instead of being amorphous. For (Sm0.5Gd0.5)2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 and (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7, phase separation appears after irradiation. The results from the first principles calculations confirm that Gd2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 exhibits stronger radiation resistance than the corresponding single pyrochlores, which agrees with the irradiation experiment result. This study demonstrates that the high-entropy pyrochlores exhibit excellent irradiation stability and can be considered as the potential matrix material for immobilization of high-level nuclear waste.

Order–disorder transition and thermal conductivities of the (NdSmEuGd)(1-x)/2Dy2xZr2O7 series

Rare-earth zirconates with pyrochlore and fluorite structures have recently been identified as promising thermal barrier coating materials owing to their low thermal conductivities. In this study, six samples with the general formula (NdSmEuGd)(1-x)/2Dy2xZr2O7 were synthesized to further reduce the thermal conductivity. X-ray diffraction and Raman spectroscopy showed that the transition from an ordered pyrochlore to a disordered fluorite structure is due to cation and anion disorder. Transmission electron microscopy showed that anion disorder occurred before cation disorder. A modified mass disorder parameter was introduced into this system, which can describe the change in thermal conductivity well. This parameter can be a basis for designing more complex materials with lower thermal conductivities.

Physical properties and radiation tolerance of high-entropy pyrochlores Gd2(Ti0.25Zr0.25Sn0.25Hf0.25)2O7 and individual pyrochlores Gd2X2O7 (X= Ti, Zr, Sn, Hf) from first principles calculations

The physical properties of high-entropy ceramics (HEC) of Gd2(Ti0.25Zr0.25Sn0.25Hf0.25)2O7 and Gd2X2O7 (X = Ti, Zr, Sn, Hf) are investigated using first principles method. The structural properties, anti-site defect energetics, mechanical properties and electronic properties are compared. The lattice constant of HEC is within the range of its constituent pyrochlores, and the local lattice distortion presents in HEC. The cation anti-site defect is much easier to form in HEC than in pyrochlore, which implies that HEC can easily transfer to disordered fluorite structure and exhibit stronger radiation tolerance. The “high entropy” at B site of A2B2O7-type pyrochlore had no obvious effect on mechanical properties. The electronic properties of HEC and pyrochlore are characterized by projected density of states distribution and Bader charge, and the causes of HEC lattice distortion are investigated.

High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7: A potential thermal barrier material with improved thermo-physical properties

High-entropy oxides (HEOs) are widely researched as potential materials for thermal barrier coatings (TBCs). However, the relatively low thermal expansion coefficient (TEC) of those materials severely restricts their practical application. In order to improve the poor thermal expansion property and further reduce the thermal conductivity, high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 is designed and synthesized in this work. The as-prepared multicomponent material is formed in a simple disordered fluorite structure due to the high-entropy stabilization effect. Notably, it exhibits a much higher TEC of approximately 12.0 × 10−6 K−1 compared with those of other high-entropy oxides reported in the field of TBCs. Besides, it presents prominent thermal insulation behavior with a low intrinsic thermal conductivity of 0.92 W·m−1·K−1 at 1400 °C, which can be explained by the existence of high concentration oxygen vacancies and highly disordered arrangement of multicomponent cations in the unique high-entropy configuration. Through high-temperature in-situ X-ray diffraction (XRD) measurement, this material shows excellent phase stability up to 1400 °C. Benefiting from the solid solution strengthening effect, it shows a higher hardness of 8.72 GPa than the corresponding single component compounds. The superior thermo-physical performance above enables (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 a promising TBC material.

Defining the Structural Stability Field of Disordered Fluorite Oxides.

Fluorite-structured oxides constitute an important class of materials for energy technologies. Despite their high level of structural symmetry and simplicity, these materials can accommodate atomic disorder without losing crystallinity, making them indispensable for uses in environments with high temperature, changing chemical compositions, or intense radiation fields. In this contribution, we present a set of simple rules that predict whether a compound may adopt a disordered fluorite structure. This approach is closely aligned with Pauling’s rules for ionic crystal structures and Goldschmidt’s rules for ionic substitution.

Antimony substitution leading to structural transformation (Bixbyite→ Fluorite) and altering the optical band gap in Y2O3

The transformation of ordered bixbyite structured Y2O3 to cation disordered fluorite structure is demonstrated by antimony substitution in the present work. The samples are prepared by the wet chemical method and characterized by employing various physicochemical techniques. The superstructure reflections of bixbyite were observed for samples prepared with 10 mol% of antimony, beyond which the structure transformed into a disordered fluorite structure. The diffracted peaks became broad in the powder X-ray diffraction pattern with increasing antimony concentration. The Williamson-Hall analysis indicated compressive strain in antimony substituted samples. Microscopy and Raman spectral measurements confirmed this structural transformation. The antimony existed in the III oxidation state, as revealed by XPS measurements. The optical bandgap of Y2O3 changed from 4.81 to 3.62, and 3.20 eV for 20 and 50 mol% antimony substituted samples. In the photoluminescence emission spectrum, the transitions of the excited Jahn-Teller state of Sb3+ are observed in the visible region. 50 mol% antimony substituted yttria sample catalyzed the complete degradation of aqueous rhodamine-6G (Rh-6G) in 105 min, assisted by the UV–visible radiation. The reaction followed pseudo-first-order kinetics, and the catalyst showed nearly the same efficiency up to five times of its reuse. The recovered catalyst did not undergo any change in its structure.

Sintering behavior and thermal conductivity of Y2O3 fully stabilized HfO2 ceramics

A series of Y2O3 fully stabilized HfO2 ceramics (Hf1−xYxO2−0.5x, x = 0.20, 0.24, 0.28, 0.32, 0.36 and 0.40) were synthesized by solid-state reaction at 1500 °C. The phase composition, thermal conductivity and sintering behavior at 1400 °C of the ceramic bulks were studied. The Hf1−xYxO2−0.5x ceramics were comprised of pure cubic phase with disordered fluorite structure. No phase transformation occurred in the ceramics with Y3+ doping concentration ranging from 0.24 to 0.36. Both Hf0.64Y0.36O1.82 and Hf0.6Y0.4O1.8 samples revealed the lowest grain growth rate when sintered at 1400 °C, indicating good resistance to sintering. However, the Vicker’s hardness and Young’s modulus after annealed for 96 h at 1400 °C for the Hf0.64Y0.36O1.82 sample were 8.97 and 200 GPa, respectively, much lower than those of the Hf0.6Y0.4O1.8 (14.51 and 237 GPa, respectively). The thermal conductivity of Hf1−xYxO2−0.5x first decreased and then increased slowly as the Y3+ content increased, and the associated mechanism was discussed.

Ordered to defect fluorite structural transition in Ce1-xNdxO2-δ system and its influence on ionic conductivity

Abstract The structural transformation from fluorite – defect fluorite – hexagonal phase on the heavily doped ceria system (Ce1-xNdxO2-δ X = 0, 25, 50, 75 and 100 mol%) has been investigated in this study. The oxygen ionic conductivity increased with dopant content till 50 mol% due to the formation of rare earth C-type superstructure defect fluorite. The co-formation of rare earth C-type structure along with the fluorite increase the anion vacancies and facilitates the barrier free ion migration. The long-range ordering and the existence of mixed phases in the heavily doped (75 mol%) system lowered the oxide ion conductivity. Ce0.5Nd0.5O1.75 in disordered fluorite structure exhibits a superior ionic conductivity of 6.0 mS/cm with an activation energy of 0.93 eV which has a potential as electrolyte in solid oxide fuel cells application.

Local Disorder in Ln2Ti2O7 (Ln = Gd, Tb, Dy) Pyrochlores

The temperature dependences of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectra of Ln2Ti2O7 (Ln = Gd, Tb, Dy) geometrically frustrated magnets with a pyrochlore structure are presented. The temperature dependences of the Ln-O and Ti-O interatomic bond lengths and Debye-Waller factors σ2 characterizing the local static and dynamic disordering of bonds have been obtained in the temperature range of 10–300 K. The stiffnesses of metal-oxygen bonds and the parameters of local disordering of the structure have been determined from the dependences σ2(T). The effect of the magnetic state of rare-earth ions on local features of the Ln2Ti2O7 structure has been described. The characteristics of local disorder in pyrochlores under study have been compared to dysprosium zirconate Dy2Zr2O7 having a disordered fluorite structure.

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

Abstract A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.

Potential Benefit of Amorphization in the Retention of Gaseous Species in Irradiated Pyrochlores

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with B=Ti amorphize much more readily than B=Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

Potential benefit of amorphization in the retention of gaseous species in irradiated pyrochlores

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with BTi amorphize much more readily than BZr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

Structural and magnetic short-range order in fluorite Yb2TiO5

We studied structural and magnetic ordering in ${\mathrm{Yb}}_{2}{\mathrm{TiO}}_{5}$ using synchrotron x-ray diffraction, neutron diffraction and total scattering, ac and dc susceptibility, and inelastic neutron scattering. Diffraction measurements reveal an average disordered fluorite structure with additional diffuse scattering features, which are caused by structural short-range orthorhombic order, as evidenced by the neutron pair distribution function measurements. The ac susceptibility measurements show a broad peak at ${T}_{f}\ensuremath{\approx}0.35$ K that displays Arrhenius behavior with an activation energy of 2.51(5) meV. Zero-field neutron scattering measurements show broad magnetic diffuse scattering in the elastic channel with an antiferromagnetic-type gapless excitation extending to 1.5 meV. A polarized state with partial spin order is induced with an applied magnetic field which opens a gapped excitation that increases monotonically with field strength.

Helium ions’ irradiation effects on Gd2Zr2O7 ceramics holding complex simulated radionuclides

A series of (Gd,A)2(Zr,B)2O7 samples bearing various amounts of complex TRPO waste (0, 25 and 40 wt%) were irradiated with 0.5 MeV He2+ ions at fluences ranging from 1 × 1014 to 1 × 1017 ions/cm2. All the irradiated samples transformed from pyrochlore to disordered fluorite structure with micro-swelling. The irradiation tolerance of the (Gd,A)2(Zr,B)2O7 solid solutions weakened with increased TRPO waste content. However, no obvious amorphization was observed even in ceramics bearing the highest amount of TRPO waste. Raman spectroscopy revealed that the vibration frequencies of the atomic bonds changed, and the bands became broader due to irradiation induced disorder.

Transparent glass–ceramics with (Eu3+,Yb3+):YNbO4 nanocrystals: Crystallization, structure, optical spectroscopy and cooperative upconversion

In the present work, we report on a comprehensive study of crystallization, structure and optical spectroscopy of transparent glass–ceramics with (Eu3+,Yb3+):YNbO4 nanocrystals synthesized by secondary heat-treatments of glass of the Li2O–Al2O3–SiO2 system, for the first time, to the best of our knowledge. Heat treatments result in volume crystallization of RENbO4 with the sizes of 4–15nm. Crystals of rare-earth niobates with disordered fluorite structure (T′) appear during heat-treatment at 720–740°C for 6h, crystals with tetragonal structure (T) appear at higher temperatures or longer durations of heat-treatment, and in samples heat-treated at 1000°C, the monoclinic form (M) is detected additionally. Rare-earth niobates act as nucleating agents for bulk crystallization of β-quartz solid solutions, the main crystalline phase of the glass–ceramics, which ensures their good thermo-mechanical properties. Optical spectroscopy confirms entering of both Eu3+ and Yb3+ ions into the RENbO4 nanophase and their specroscopic properties variation according to the T′→T→M phase transformations. Under UV excitation, glass–ceramics heat-treated at 900°C provide intense red emission with the color coordinates x=0.665, y=0.335 (CIE 1931). In the sample, heat-treated at 1000°C, a partial reduction of Eu3+ to Eu2+ is observed which allows for tuning the color properties of emission. When excited in the near-IR by an InGaAs diode, the initial glass and glass–ceramics show red cooperative upconversion due to the 2Yb3+→Eu3+ energy transfer. The efficiency of the latter is ~10%. The developed materials due to the good emission and thermo-mechanical properties are promising for the development of color-tunable red phosphors.

Thermophysical properties of pyrochlore and fluorite phases in the Ln2Zr2O7–Y2O3 systems (Ln = La, Nd, Sm): 2. Comparison of conventionally sintered and SPS samples in the systems Nd2Zr2O7–Y2O3 and Sm2Zr2O7–Y2O3

Thermal diffusivities of pyrochlore and fluorite phases in the Nd2Zr2O7–Y2O3 and Sm2Zr2O7–Y2O3 systems were measured using the laser flash method for the first time. Samples synthesized by co-precipitation were sintered by conventional sintering at 1600°C or by spark plasma sintering/field assisted sintering technique (SPS). Thermal conductivity was calculated based on thermal diffusivity data, heat capacity data measured by differential scanning calorimetry (DSC) and densities measured using the Archimedes method. Thermal diffusivity and conductivity in the Nd2Zr2O7–Y2O3 system demonstrated similar dependencies on the Y2O3 content as were observed for the La2Zr2O7–Y2O3 system: the highest values were indicated for most ordered Nd2Zr2O4 pyrochlore whereas the lowest were observed for the disordered fluorite structure containing more Y2O3 than pyrochlore. However the thermal diffusivity and conductivity dependencies in the Sm2Zr2O7–Y2O3 were different – the highest values were measured for fluorite phase having higher Y2O3 content.

Ion irradiation induced order-to-disorder transformation in δ-phase Lu4Hf3O12

In this study, polycrystalline δ-phase Lu4Hf3O12 was irradiated with 6MeV Xe26+ ions to fluences ranging from 2×1013 to 1×1015ions/cm2. Ion irradiation-induced microstructural evolution was examined by using grazing incidence X-ray diffraction (GIXRD). A complete phase transformation from ordered rhombohedral to disordered fluorite (O–D) was observed by a fluence of 1×1015ions/cm2, equivalent to a peak ballistic damage dose of ∼3.49 displacements per atom (dpa). To research the different irradiation effect between light ion and heavy ion on δ-Lu4Hf3O12, 400keV Ne2+ ions were implanted to ion fluences ranging from 1×1014 to 1×1015ions/cm2. A complete O–D crystal structure transformation was observed by a fluence of 5×1014ions/cm2 (∼0.22dpa). This threshold dose was found to be observably lower than the threshold dose to produce order-to-disorder transformation using Xe26+ ions on δ-Lu4Hf3O12. This suggests that heavy ions are less efficient than light ions in producing the retained defects that are responsible for the O–D transformation. The theoretical calculations show that the O–D transformation of δ-phase was attributed to the anion oxygen Frenkel pair defect. The ion irradiation-induced transformation of δ-phase Lu4Hf3O12 into disordered fluorite structure observed here is also discussed in relation to the temperature–composition (T–C) phase diagrams for the compound.