Single Pyrochlore Phase Research Articles

Design and synthesis of high-entropy A2B2O7-type pyrochlore ceramics for the immobilization of molten salt radwastes

Pyrochlore-based high-entropy ceramics can encapsulate multiple elements with different valences, making them promising for immobilizing high-level radioactive nuclides in molten salt radwaste. This approach addresses key challenges in dry reprocessing and molten salt reactor waste management. In this study, a single-phase pyrochlore-based high-entropy ceramic is synthesized to incorporate NdF3 and CeF3 as simulated molten salt radwastes. Various synthesis components have been explored, and the ceramics were characterized using x-ray diffraction (XRD) and Raman spectroscopy. Comparative analysis revealed that the size disorder is the critical factor to synthesize single-phase pyrochlore-based high-entropy ceramics. The effects of elemental ratio and the presence of variable valence elements on the synthesis of single-phase pyrochlore-based high-entropy ceramics are elucidated through the concept of size disorder. Two single-phase pyrochlore compositions have been successfully synthesized: #HE4 (Nd2(Ti0.25Zr0.25Hf0.25Sn0.25)2O7) and #HECe0.1 (Nd2(Zr0.3Hf0.2Sn0.2Nb0.2Ce0.1)2O7). These samples were tested with a 7-day leaching experiment using the Product Consistency Test Method PCT-B. The normalized release rates of all elements in #HE4 ranged from 10-7 to 10-9 g·m-2·d-1, indicating excellent chemical durability. These findings suggest that medium- and high-entropy pyrochlore-based ceramics are effective for immobilizing radioactive nuclides in molten salt radwastes.

Influence of synergistic effect on the structure and dielectric property of La2(TiZrSnHfGe)2O7 high entropy oxides

In this study, La2(TiZrSnHfGe)2O7 high entropy oxides were firstly prepared by solid state reaction method. The effects of different sintering temperatures on the structure, morphology and dielectric properties were studied. When the pre-calcination temperature is 1300 °C, the powder exhibits a single pyrochlore phase instead of defective fluorite phase. The existence of high entropy affects the changes in the crystal structure of A2B2O7 type oxides, resulting in lattice distortion. The dielectric results showed that the ceramics have certain frequency dispersion, and the dielectric properties are affected by temperature and frequency. Due to the high entropy effect, the La2(TiZrSnHfGe)2O7 ceramic exhibits good dielectric stability at both high and low frequencies. The formation of defect dipole clusters in the system results in the increase of dielectric constant. It is feasible to change the dielectric properties of ceramics by designing high entropy oxides, which showed the potential value in microelectronics field.

Potential of Y2Sn2O7:Eu3+, Dy3+ Inorganic Nanophosphors in Latent Fingermark Detection

In this work, we investigated the potential of Eu3+/Dy3+-codoped Y2Sn2O7 fluorescent nanophosphors to visualize latent fingermarks. We prepared these nanophosphors with various doping concentrations by the conventional coprecipitation reaction. The crystal structure, morphology, luminescence properties, and energy transfer mechanisms were studied. The crystalline phase was characterized by X-ray diffraction and crystal structure refinement using the Rietveld method. XRD measurements showed that the samples crystallized in the pure single pyrochlore phase with few more peaks originated from secondary phases and impurities generated during phosphor production, and that Eu3+ ions occupied D3d symmetry sites. The average crystallite size after mechanical grinding was less than 100 nm for all compositions. The optical characterization showed that, when excited under 532 nm, the Eu3+/Dy3+-codoped Y2Sn2O7 samples’ main intense emission peaks were located at 580–707 nm, corresponding to the 5D0→7Fj (j = l, 2, 3, and 4) transitions of europium. In fact, the 5D0→7F2 hypersensitive transition is strongly dependent on the local environment and was quite weak in Eu3+:Y2Sn2O7 at low Eu3+ doping levels. We found that the presence of Dy3+ as a codopant permitted enhancing the emission from this transition. The calculated PL CIE coordinates for the synthesized nanophosphors were very close to those of the reddish-orange region and only slightly dependent on the doping level. Various surfaces, including difficult ones (wood and ceramic), were successfully tested for latent fingerprint development with the prepared Eu3+/Dy3+-codoped Y2Sn2O7 fluorescent nanophosphor powder. Thanks to the high contrast obtained, fingerprint ridge patterns at all three levels were highlighted: core (level 1) islands, bifurcation, and enclosure (level 2), and even sweat pores (level 3).

One-step synthesis of single-phase, bulk Bi2Ti2O7 pyrochlores by deep undercooling rapid solidification and its formation mechanism

Preparation of dense Bi2Ti2O7 ceramics with single-phase pyrochlore structure is challenging using the conventional sintering method due to its thermal instability at ≥ 650 °C. Here we synthesized dense and phase-pure Bi2Ti2O7 ceramics from a mixture of Bi2O3 and TiO2 by a novel deep undercooling rapid solidification method using the aerodynamic levitation technique. We reported a surprising discovery that the pyrochlore phase can exist with Bi/Ti ratio varying between 0.9 and 1.1. By a comprehensive investigation on the phase purity, crystal structure, oxidation states and electrical properties, we revealed that the pyrochlore structure is primarily stabilized by the solidification process rather than presence of a large amount of Ti3+. With the advantages of being containerless, one-step and fast processing, deep undercooling rapid solidification is an effective and efficient approach to prepare dense Bi2Ti2O7 pyrochlores, and can be expanded to other thermally unstable compounds to obtain bulk materials.

Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.

Design, synthesis, and influencing factors of medium-/high-entropy Y2(ZrTiGeHfSnSi)2O7 with a pyrochlore structure

A set of medium-/high-entropy pyrochlore oxides (M-HEPOs) with different compositions were designed and synthesized. The phase compositions of the prepared materials were determined through X-ray diffraction and Raman spectroscopy. Among the 24 designed samples, two high-entropy and six medium-entropy samples formed single-phase pyrochlore materials, two medium-entropy samples formed dual-phase pyrochlore materials, and the others formed multiphase materials dominated by the pyrochlore phase. The factors affecting the formation of single-phase pyrochlore structure were studied. The results revealed that size disorder is a major element determining the formation of single-phase pyrochlore, whereas mixing entropy has minimal effect. It is anticipated that this study will provide significant insights into the effects of size disorder and mixing entropy on the formation of M-HEPOs.

Structural evolution and chemical durability of uranium-doped high-entropy pyrochlore

The high-entropy ceramic solidified body ((LaPrEuGdYb)yUx)2Zr2O7+δ (y = (1−x)/5, x = 0.05, 0.10, 0.15, 0.20, 0.25) was prepared by immobilizing uranium (U) on the A-site of the pyrochlore in this study. The phase composition and evolution of solidified body were determined by XRD, Raman and SEM analysis. The phase of (LaPrEuGdYb)yUx)2Zr2O7+δ ceramic was varied with U doping amounts: a dual-phase of pyrochlore and fluorite with no U doping. A single-phase pyrochlore with U doping amounts from x = 0.15 to x = 0.20, and the U3O8 phase was precipitated at doping levels above x = 0.25. The solid solution limit for high entropy pyrochlore in relation to uranium is roughly x = 0.2. The results should be linked to the increase in configurational entropy and the variation of U valence in the ceramic bodies, as evidenced by XPS. The normalized leaching rate of U from high-entropy ceramic waste forms (x = 0.20) was about 1.65 × 10−5 g/m2⋅d after 42 d leaching. It performed well in the leaching tests. Our study is expected to provide candidate materials for solidifying complex high-level radioactive waste (HLW).

Sintering resistance and phase stability of (Y0.2La0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 for ultra-high temperature thermal barrier coatings

High-temperature stability, including sintering resistance and phase stability, is critical for thermal barrier coatings (TBCs) applied at ultra-high temperature over 1500 °C. In this work, we reported a high-entropy rare-earth zirconate (Y02La0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 (YLNSE) synthesized by solid-state reaction method. X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive spectrometer (EDS) mapping results confirmed that YLNSE formed a single-phase pyrochlore structure with uniform rare earth elements distribution. YLNSE maintained more uniform microstructure, higher porosity retention, lower grain growth rate and better thermomechanical properties during sintering at 1500 °C, as the results of the ordered crystal structure and sluggish diffusion effect, indicating a better sintering resistance than conventional TBC material yttria-stabilized zirconia (YSZ). In addition, YLNSE also demonstrated high coefficient of thermal expansion (CTE, 11.04 × 10-6K-1, 25–1500 °C) and low thermal conductivity (1.95 W m-1 K-1, 1500 °C), as well as good phase stability at 1500 °C. In view of the excellent high-temperature stability of YLNSE, it is a promising candidate for ultra-high temperature TBCs.

High-capacity isomorphic immobilization and leaching mechanism of Gd2Zr2O7-based ceramic waste form

Gd2Zr2O7(GZO)-based ceramic waste form has attracted broad interest due to its potential applications in the long-term disposal of high-level nuclear waste. However, its high-capacity isomorphic immobilization and leaching mechanism are not well understood yet. Herein, GZO-based waste form with a single pyrochlore phase and homogeneous element distribution, achieving an isomorphic waste load as high as 50 wt%, was fabricated successfully. Isomorphism failure of the waste form is due to the Ln6MoO12 (Ln is lanthanides) formation and direct precipitation of PdO from the oversaturated solid solution. Chemical durability tests showed that the 42-day leaching rates of nearly all elements in the waste form with the maximum waste load were only between 10−6 and 10−5 gm−2d−1. The leaching of 0.5SW·0.5GZO over a period of 42 days is primarily controlled by dissolution, leading to a gradual restructuring of the surface in two distinct stages: the formation of an amorphous passivation film and the agglomeration of amorphous precipitates. Notably, the formation of the amorphous passivation film stands as the primary factor influencing the leaching rate of the waste form.

Site Disorder as a Predictor for Compositionally Complex 5RE2Zr2O7 Ceramic Phase Stability

AbstractPhase formation and stability of five component compositionally complex rare earth zirconates (5RE2Zr2O7) were investigated by X‐ray diffraction and electron microprobe analysis. Zirconates with different rare earth compositions (LaNdSmEuDy, LaNdSmEuYb, LaNdEuErYb, LaNdDyErYb, SmEuDyYHo, LaYHoErYb, and DyYHoErYb) were synthesized at 1700°C and 2000°C by the solid‐state method to investigate the effect of A‐site site disorder (δA) on phase stability. Increased site disorder results from mixed cation occupancy with localized crystallographic strain and bond disorder. Compositions LaNdSmEuDy (δA = 4.6) and LaNdSmEuYb (δA = 6.0) produced a single pyrochlore phase and compositions SmDyYHoErYb (δA = 2.8), LaYHoErYb (δA = 6.2), and DyYHoErYb (δA = 1.7) produced a single fluorite phase. High δA compositions LaNdEuErYb (δA = 6.9) and LaNdDyErYb (δA = 7.2) produced a pyrochlore and fluorite phase mixture at 1700°C. Single phase was obtained for the latter composition at 2000°C. Of the single phase compositions calcined at 1700°C, LaNdSmEuYb and LaYHoErYb (both with largest δA) showed decomposition to mixed fluorite and pyrochlore phases during lower temperature anneals, indicating entropic stabilization. Comparison with prior work shows a temperature dependence of the critical δA for phase stability, and compositions near it are expected to be entropy stabilized.

Phase Formation of Co and Cr Co-Doped Bismuth Niobate with Pyrochlore Structure

The formation mechanisms of pyrochlore-type Bi2Co1/2Cr1/2Nb2O9+Δ (space group Fd-3m, a = 10.4838(8) Å), in the temperature range from 400 to 1050 °C were studied by employing X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. An extensive reaction between the binary metal oxides was found to begin at temperatures above 550 °C, following the transition of monoclinic α-Bi2O3 into a tetragonal β-Bi2O3 polymorph. The synthesis process occurs in several stages when Bi-rich intermediate products (Bi6CrO12, Bi6Cr2O15, and Bi5Nb3O15) transform into bismuth-depleted BiNbO4 and a chromium–cobalt spinel is formed. The formation of a single pyrochlore phase occurs at the final reaction stage at 1050 °C via the doping of bismuth ortho-niobate, BiNbO4, by the transition metal cations. The observed mechanism is essentially similar to the mechanism of tantalate-based phases except for the formation of Bi5Nb3O15 at the intermediate reaction stages.

Immobilization and He+ irradiation effects of Gd2Zr2O7-based ceramic waste form for radioactive waste disposal

The Gd2Zr2O7(GZO)-based ceramic waste form exhibits promising potential for radioactive waste immobilization. However, the isomorphic immobilization capacity and irradiation effects of GZO-based ceramic waste forms with varying waste loads remain inadequately understood. This study employed a facile combustion method to synthesize ceramic powders and utilized conventional sintering with a relatively short dwelling time to fabricate GZO-based ceramic waste forms. We investigated the isomorphic immobilization behavior and He ion irradiation effects of the synthesized waste forms. The outcomes demonstrate that GZO effectively immobilizes simulated waste composed of 11 types of oxides, achieving a waste load as high as 50 wt% with a single pyrochlore phase and homogeneous element distribution. Under He+ irradiation, aggravated structural degradations, including exacerbated lattice swelling, increased amorphization degree, and premature He bubble chain/microcrack formation, were observed in GZO-based waste forms with an increasing waste load.

Effect of doping rare earth oxides (RE= Sc, Y, Gd, Eu) on structural, thermal and mechanical properties of La2(Zr0.7Ce0.3)2O7 ceramics

Rare-earth (RE) oxide-doped La2(Zr0.7Ce0.3)2O7 (LZC) has attracted great interest in thermal barrier coatings (TBCs) because of its lower thermal conductivity. However, studies on the laws governing the effect of different types of RE oxide doping on the performance of LZC ceramics are limited. Therefore, in this study, different types of RE (Sc, Y, Gd and Eu) doped LZC ceramic bulks have been prepared using solid phase reaction method. The effects of RE doping on the crystal structure, mechanical properties and thermal properties of the LZC ceramics were investigated. The results indicated that all five ceramics exhibited a single pyrochlore phase structure, whereas their lattice constants were reduced after doping, with (La0.85Sc0.15)2(Zr0.7Ce0.3)2O7 having the lowest lattice constant. The effect of rare earth doping on the grain growth of LZC was not evident; however, it could significantly improve the hardness of LZC. In addition, doping with all four RE ions significantly reduced the thermal conductivity of the LZC ceramics, and the most significant reduction was observed for Gd3+ doping.

Multicomponent solid solution with pyrochlore structure

Multicomponent oxide with pyrochlore structure (A2B2O7), containing 7 different A-site cations and 3 B-site cations in equiatomic amounts, was synthesized. Powders with nominal composition (La1/7Sm1/7Nd1/7Pr1/7Y1/7Gd1/7Yb1/7)2(Sn1/3Hf1/3Zr1/3)2O7 were fabricated through a reaction of metal nitrates (A-site) and metal chlorides (B-site) with sodium hydroxide during the solid state displacement reaction. Room temperature synthesis initially resulted in the obtainment of amorphous powders, which crystallized after subsequent calcination to form single crystalline compounds. Crystalline high-entropy ceramic powders formation took place at temperatures as low as 750°C. During calcination, defective fluorite (F-A2B2O7) and crystal pyrochlore (Py-A2B2O7) structures coexist. A large number of cations induce the obtainment of stable high-entropy pyrochlores. Results showed that sintering at 1650°C lead to pure crystalline single-phase pyrochlore formation. High-density ceramic, free of additives, was obtained after powders were compacted and subjected to pressureless sintering at 1650°C. Multicomponent pyrochlore structure was investigated using the theoretical and experimental multi-methodological approach.

Heavily doped high-entropy A2B2O7 pyrochlore

A novel class of high-entropy pyrochlore compounds with multiple elements at the A and B site positions (A2B2O7) was successfully obtained. Powders with (La1/7Sm1/7Nd1/7Pr1/7Y1/7Gd1/7Yb1/7)2(Sn1/3Hf1/3Zr1/3)2O7 nominal composition were fabricated from pure metal oxides obtained through a reaction of metal nitrates (for site A) and metal chlorides (for site B) with sodium hydroxide during the solid-state displacement reaction (SSDR). The phase evolution was analyzed using XRD method. During the thermal treatment of ten individual metal oxides, the single pyrochlore phase was created. The present study showed that the highdensity (98%TD) ceramics with a hardness of 8.1GPa was successfully obtained after pressureless sintering at 1650 ?C for 4 h. Results of the Raman study and the Rietveld structural refinement showed that sintered highentropy ceramics is characterized by a single-phase pyrochlore structure, which was investigated in detail.

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.

Synthesis and characterization of sol-gel derived lanthanum zirconate ceramic aerogels toward ultralow thermal conductivity

La2Zr2O7 ceramic aerogels (LZO CAs) were successfully fabricated via sol-gel method followed by heat treatment process. The morphology, microstructure, and phase composition of the as-prepared samples were systematically studied using various technological means. The results indicated that the as-prepared LZO CAs present a typical three-dimentional (3-D) porous network with a single-phase pyrochlore crystal structure. In addition, the as-prepared LZO CAs presented high specific surface area of 594.85 m2/g, low thermal conductivity of 0.033 W/(m·K) at room temperature, and remarkable high temperature heat-insulating property. This work offers a novel strategy for the development of other rare-earth zirconates ceramic aerogels which can be used as high temperature insulations.

Phase Evolution in the CaZrTi2O7-Dy2Ti2O7 System: A Potential Host Phase for Minor Actinide Immobilization.

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy3+ incorporation within the self-charge balancing zirconolite Ca1–xZr1–xDy2xTi2O7 solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L3-edge XANES region confirmed that Dy was present uniformly as Dy3+, remaining analogous to Am3+. Fitting of the EXAFS region was consistent with Dy3+ cations distributed across both Ca2+ and Zr4+ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy3+ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi2O7–REE3+Ti2O7 binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE3+ cations.

Eu-Doped Pyrochlore Crystal Nano-Powders as Fluorescent Solid for Fingerprint Visualization and for Anti-Counterfeiting Applications.

Undoped Y2Sn2O7 and Eu3+ doped Y2Sn2O7 samples with doping concentrations 7%, 8%, 9%, and 10% are successfully synthesized by the co-precipitation method. A complete structural, morphological, and spectroscopic characterization is carried out. XRD measurements reveal that samples crystallize in the pure single pyrochlore phase and Eu3+ ions occupy sites with D3d symmetry. After mechanical grinding, the average crystallite size is less than 100 nm for all compositions. Optical characterization shows emission from the 5D0 level towards the lower lying 7F0,1,2,3,4 levels. The CIE color coordinates of all the pyrochlore phosphors are very close to those of the ideal red light. For the visualization of latent fingerprints, different surfaces are tested, including difficult ones (wood and ceramic), with excellent results. All three levels of fingerprint ridge patterns are visualized: core (Level 1), bifurcation and termination (Level 2), and sweat pores (Level 3). Moreover, our nano-powders are used to prepare a stable fluorescent ink.

Microstructure and electrical properties of new high-entropy rare-earth zirconates

In the present work, a series of new high-entropy rare-earth zirconates were prepared by a solid state reaction method. The crystal structure, microstructure and electrical properties of high-entropy rare-earth zirconates were characterized by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and electrochemical impedance spectroscopy. All the high-entropy rare-earth zirconates have a single-phase pyrochlore structure with a relative density of more than 95.7%. The grain sizes of high-entropy rare-earth zirconates are 1.3–2.4 µm, which are finer than low-entropy rare-earth zirconates prepared by the same process. (Nd0.2Sm0.2Eu0.2Gd0.2Dy0.2)2Zr2O7 has the excellent electrical conductivity of 7.1 × 10–3 S·cm–1 at 1073 K.