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Zirconate pyrochlores have been extensively studied as candidate waste forms for the immobilization of actinide wastes due to their flexible crystal chemistry, excellent structural stability, and exceptional radiation tolerance. Herein, we report the synthesis of a series of compounds Eu2Zr2–xO7–2x (x = 0–1, step = 0.125) using a co‐precipitation route to form solid solutions between the two end members, Eu2Zr2O7 pyrochlore and Eu2ZrO5 defect fluorite. It is important to understand the pyrochlore to defect fluorite transformation as a function of Zr/Ln molar ratio. The structural investigation by x‐ray diffraction has shown the increasing cation disorder and corresponding structural transitions from the ordered pyrochlore to disordered pyrochlore, then to defect fluorite. While diffuse reflectance spectroscopy provides similar optical absorption features reflecting the presence of Eu(III) ion, Raman spectroscopy has revealed more details on the phase transition and short‐range structures. This work highlights the structural flexibility in zirconate materials as potential nuclear waste forms for the immobilization of actinide‐rich waste streams, especially suitable for the separated minor actinides. [ABSTRACT FROM AUTHOR]

Y2Ti2−xSnxO7 (x = 0–2) pyrochlore sodium aluminoborosilicate glass–ceramics (GCs) are produced by calcining the pelletized Y–Ti–Sn oxide mixture and glass precursor at 1200 or 1300°C for 4 h. The metal oxide mixture is prepared by a soft chemistry route. X‐ray diffraction, Raman spectroscopy, and electron microscopy are employed to investigate the formation of pyrochlore GCs and local crystal structures. Near phase pure Ti‐rich pyrochlores are produced with minor phase SnO2 observed for Sn‐rich materials. The cell parameters of the pyrochlore structures refined by Le Bail fitting are in good agreement with the published data and increase linearly with the gradual increment of Sn substitution. With progressively increasing Sn proportion on pyrochlore B‐site, Raman characteristic bands of the pyrochlore structure become sharper and well defined. The Raman A1g peak position and its full width at half‐maximum are linearly progressed with increasing x (Sn). The presence of the melting glass facilitates the pyrochlore formation, with ceramic grain sizes ranging from submicron to microns. Transmission electron microscopy and selected area electron diffraction observations indicate the sample possesses a relatively high crystallographic perfection at the atomic level. This new series of pyrochlore GCs and the method disclosed herein may pave the way for further materials development as potential nuclear waste forms. [ABSTRACT FROM AUTHOR]

Glass-Ln 2 Ti 2 O 7 pyrochlore (Ln = Gd, Tb, Er, Yb) was fabricated by sintering the pelletized mixture of glass precursor and Ln-Ti composite at 1200 °C. The phase pure pyrochlore was in - situ crystallized in the amorphous glass matrix. The Ln-Ti composite was prepared by a simple soft chemistry route in an aqueous solution to ensure the homogeneity of the product. Thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy techniques were employed to investigate the glass-Ln 2 Ti 2 O 7 pyrochlore structure formation. The cell parameters for the pyrochlore structures by Le Bail fitting are in good agreement with the published data. The presence of the melting glass matrix facilitates the pyrochlore formation, with particle sizes in the range of 1–3 μm. This new aqueous synthetic method provides a simple pathway to produce glass-Ln 2 Ti 2 O 7 pyrochlore without using organic solvent and/or milling procedures, making it an attractive potential method for scale-up production. [ABSTRACT FROM AUTHOR]

Ln2Ti2O7 (Ln=Y and Gd) pyrochlore glass-ceramics have been fabricated successfully via internal crystallization. Subsequently, the phase evolution from Ln2Ti2O7 pyrochlores to Ln0.5U0.5Ti2O6 brannerites in glass with uranium (U) substitutions on the Ln-site of Ln2Ti2O7 has been investigated using X-ray diffraction, scanning electron microscope-electron dispersive spectroscopy, transmission electron microscopy, Raman and diffuse reflectance spectroscopy. Combined characterization by XRD, SEM- EDS and TEM SAED confirms the structures and phase evolution while Raman spectroscopy reveals characteristic vibration modes for both pyrochlore and brannerite. In addition, DRS of the U5+ ion has been used to probe the phase evolution, with the corresponding f- f transition band of 2 F7/2 energy level significantly shifting to longer wavenumbers due to the local coordination environment changing from eightfold coordination in pyrochlore to sixfold coordination in brannerite. [ABSTRACT FROM AUTHOR]

• Lanthanide titanate stannate (Ln 2 TiSnO 7) pyrochlore glass-ceramics are synthesized. • Cell parameters are in good agreement with the calculated data. • There is a linear relationship between Raman A 1 g peak position and Ln3+ionic radii. • Average pyrochlore grains are in submicron size. Ln 2 TiSnO 7 (Ln = Nd-Er) pyrochlore glass-ceramics are produced by calcining the pelletized oxide mixture and sodium aluminoborosilicate glass precursor at 1200 °C for 4 h. The ceramic Ln -Ti-Sn oxide mixture is prepared by a soft chemistry route to enhance reactant reactivity and product homogeneity. X-ray diffraction, Raman spectroscopy, and electron microscopy techniques are used to confirm the formation of the pyrochlore structure in glass. The cell parameters of the pyrochlores by Le Bail fitting are consistent with the calculated data. The variation of the lanthanide on pyrochlore A -site has insignificant effect on the Raman F 2 g mode ∼ 310 cm−1, but has an apparent influence on A 1 g mode ∼ 504–512 cm−1, which gradually shifts to low frequencies with increasing Ln 3+ionic radii. The average sizes of the pyrochlore crystals are ∼ 0.31–0.62 μm, which are uniformly distributed in glass phase. These Ln 2 TiSnO 7 pyrochlore glass-ceramics with mixed elements on B -site, possessing high radiation tolerance feature due to the presence of Sn, could be used as nuclear waste forms. [Display omitted] [ABSTRACT FROM AUTHOR]

Pyrochlore Nd 2 Zr 2 O 7 nanoparticles were prepared by complex-precipitation in aqueous media, followed by calcination in MgO matrix and subsequent dissolution processing. A suite of characterization techniques, including X-ray diffraction, Raman, TEM, SEM, dynamic light scattering, and nitrogen sorption, was employed to investigate the structure and particle size of the synthesized nano materials. Results show that calcination at 1200 °C for 20 h forms Nd 2 Zr 2 O 7 with pyrochlore structure. The matrix phase (MgO) had no effect on the formation of pyrochlore phase. The MgO phase was readily removed by dissolution at 0.5 M HNO 3 aqueous solution; and the remaining pyrochlore Nd 2 Zr 2 O 7 nanoparticles had a diameter of approximately 200 nm estimated by TEM and approximately 550 nm determined by light scattering due to slight aggregation. The bulk density of the pelletized powder reached approximately 99% of theoretical value, after uniaxial pressing at 2.0–2.5 MPa and sintering at 1400 °C for 48 h. [ABSTRACT FROM AUTHOR]

Pyrochlore structured Gd2Zr2O7 and Nd2Zr2O7 are produced via complex precipitation processing. A suite of characterization techniques, including FTIR, Raman, X-ray and electron diffraction, TEM, SEM as well as nitrogen sorption are employed to investigate the structural and grain size evolution of the synthesized and calcined powder. Results show that Gd2Zr2O7 with the pyrochlore structure are produced after calcination at 1400°C for 12h while Nd2Zr2O7 has already formed the pyrochlore structure at 1200°C. This method allows the formation of dense materials at relatively low temperature, with bulk densities over 92% of the theoretical values achieved after sintering at 1400°C for 50h. This unique aqueous synthetic method provides a simple pathway to produce pyrochlore lanthanide zirconate without using either organic solvent and/or mechanical milling procedures, making the synthesis protocol an attractive potential scale-up production of highly refractory ceramics. [ABSTRACT FROM AUTHOR]

Pyrochlore-structured lanthanide stannate ceramic ( Ln2 Sn2 O7) has been synthesized via a new complex precipitation method. A suite of characterization techniques, including FTIR, Raman, X-ray, and electron diffraction as well as nitrogen sorption were employed to investigate the structural evolution of the synthesized and calcined powder. Raman, XRD, and selected area electron diffraction results confirm the presence of the pyrochlore structure after calcination of the powder above 1200°C. TEM imaging shows fine crystallites gradually increased in size from approximately 100 nm to about 500 nm with higher calcination temperatures. Grain growth and powder densification upon increasing the calcination temperature was confirmed by nitrogen sorption results. This aqueous synthetic method provides a simple pathway for the preparation of homogeneous lanthanide stannate ceramics. [ABSTRACT FROM AUTHOR]

Lanthanum zirconate has been prepared via a new chemical synthesis method by combining sol-gel processing and complex precipitation. The synthesis was carried out in aqueous solution under ambient conditions. The synthesized powder possessed the pyrochlore superstructure upon calcination above 1200°C. A suite of characterization techniques, including FTIR, Raman, X-ray and electron diffraction, TEM, SEM, and nitrogen sorption were employed to investigate the microstructural evolution and bulk properties. Dense ceramics (>90% relative density) were obtained after sintering at 1400°C, without need for additional processing (i.e., hot or cold isostatic pressing) or any milling steps. A mechanism has been proposed that elucidates molecular assembly of this chemical synthesis method. [ABSTRACT FROM AUTHOR]

Glass-Y 2 Ti 2 O 7 pyrochlore was fabricated by sintering the mixture of glass precursor powder and (YTi)-composite which was prepared by a soft chemistry route. X-ray diffraction and Raman spectroscopy confirmed that the phase pure pyrochlore was crystallized in-situ in amorphous glass matrix at 1200 °C with dwelling time over 1 h. Pyrochlore was formed in glass matrix with cooling rate ∼0.1–40 °C/min, and independent of addition of alkali/alkaline earth fluorides. Glass matrix was able to accommodate/dissolve small amounts of impurities and the mean pyrochlore particle size was between 1 and 2 μm in glass observed by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Abstract: Pyrochlore structured Nd2Sn x Zr2−x O7 (x=0–2) ceramics were prepared by a new chemical synthesis route in an aqueous media. Raman spectra and X-ray diffraction patterns confirmed the formation of the pyrochlore structure after sintering at 1400°C. SEM results showed pelletized materials became dense with increasing Zr/Sn mole ratio and sintering temperature. These candidate materials have characteristic structures for the development of nuclear waste forms or inert matrix nuclear fuels. [Copyright &y& Elsevier]

Graphical abstract Both Y 2 Ti 2 O 7 and Gd 2 Ti 2 O 7 pyrochlore glass-ceramics have been prepared with a typical oxide route and consolidated via hot isostatic pressing, and characterized as potential waste forms for the immobilization of actinide-rich radioactive wastes. Abstract Both Y 2 Ti 2 O 7 and Gd 2 Ti 2 O 7 pyrochlore glass-ceramics (GCs) have been prepared with an oxide route and consolidated via hot isostatic pressing (HIPing) as waste forms for actinide immobilization. The crystallization of Y 2 Ti 2 O 7 /Gd 2 Ti 2 O 7 pyrochlore as the major actinide host phase in the residue glass has been confirmed with XRD, SEM-EDS and Raman spectroscopy. The static leaching tests confirmed that both GCs are chemically durable with normalized Y/Gd and Ti leach rates ∼10−4–10-5 g m-2 d-1 after 28 days. In addition, the interactions between the 316L stainless steel (SS) HIP-can and the GCs have been investigated. Interactions occur at the interface with a layer of mixed Cr/Ti oxide and CrTi 2 O 5 crystals observed for the Y 2 Ti 2 O 7 GC and a layer (∼10 μm) of Cr 2 O 3 observed for the Gd 2 Ti 2 O 7 GC as the reaction fronts from the SS cans. Furthermore, a layer (∼10 μm) of Gd-(Si/Ti)-oxide phase formed on the GC side of the interface. [ABSTRACT FROM AUTHOR]

A novel soft chemistry route was developed to synthesize Y 2 Hf x Ti 2− x O 7 (0≤ x ≤2.0) oxide solid solution. An aqueous solution containing reactants was produced to ensure the combustion reaction taking place at the molecular level. A suite of characterization techniques, including X-ray diffraction, Raman, transmission electron spectroscopy, as well as X-ray absorption near edge structure (XANES), is employed to investigate the structural and phase changes of the series. Both X-ray and electron diffraction patterns show that the Y 2 Hf x Ti 2− x O 7 system undergoes a clear composition-induced phase transition from ordered pyrochlore to disordered defect-fluorite at x ~1.5. On the other hand, Raman and XANES spectra reveal a gradual evolution of the local structure with the substitution of Hf for Ti. [ABSTRACT FROM AUTHOR]

Abstract: A series of uranium-containing gadolinium zirconate samples have been fabricated at 1450°C in oxidizing, inert and reducing atmospheres. X-ray diffraction, Raman spectroscopy and transmission electron microscopy have been utilized to confirm adoption of pyrochlore or defect fluorite structures. X-ray diffraction allowed determination of the bulk averaged structure while Raman spectroscopy and transmission electron microscopy were used to determine ordering at the microdomain scale. Diffuse reflectance, X-ray absorption near edge structure and X-ray photoelectron spectroscopies indicated a predominantly U6+ oxidation state for all the air-sintered samples, even when Ca2+ or A-site vacancies were incorporated to charge balance for U4+, a mixed U5+/U6+ oxidation state was found for samples sintered in argon, while a mixed U4+/U5+ oxidation state occurred for sintering under N2–3.5%H2. This demonstrates a degree of uranium oxidation state control through sintering conditions, and the potential of using gadolinium zirconates as host materials for uranium in nuclear waste applications. [Copyright &y& Elsevier]

MgAl 2 O 4 –Ln 2 Sn 2 O 7 (Ln = Y, La, Nd, Gd and Tb) composite materials with 50–70 vol% of MgAl 2 O 4 as potential inert matrix fuels have been synthesized using a soft chemistry route. X-ray diffraction (XRD), Raman, scanning and transmission electron microscopies (SEM/TEM) are employed to investigate the bulk and micro structures of both ceramic phases. Heat treatment at 1400 °C in air for 24 h leads to the formation of Ln 2 Sn 2 O 7 pyrochlore and MgAl 2 O 4 spinel which are confirmed by XRD, Raman and selected area electron diffraction (SAED). More importantly, the component in one phase does not affect the crystalline formation of the other. Thermal analysis results show MgAl 2 O 4 crystallizes prior to that of Y 2 Sn 2 O 7 and the crystallization of Y 2 Sn 2 O 7 delays due to the presence of MgAl 2 O 4. SEM shows uniformly distributed pyrochlore particles in spinel matrix, especially for the sample in equal volume ratio, with grain sizes being 1–5 μm for both phases. HRTEM and SAED observations confirm that both phases possess a high level of crystallographic perfection at the atomic scale. • New potential inert matrix fuel materials (MgAl 2 O 4 –Ln 2 Sn 2 O 7) are synthesised. • MgAl 2 O 4 spinel and Ln 2 Sn 2 O 7 pyrochlore are formed in one-pot synthesis. • Both spinel and pyrochlore possess high level crystallographic perfection. • 1–5 μm grains are uniformly distributed each other for equal volume cer-cer product. [ABSTRACT FROM AUTHOR]