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As part of a broader study of ceramic nuclear waste‐forms, four different lanthanide titanates were fabricated; La0.1Sm0.1Gd0.1Tb0.1Dy0.3Ho0.1Er0.2YbTiO5, Sm0.3Gd0.3Dy0.3Yb1.1TiO5, Sm0.1Gd0.4Dy0.4Yb1.1TiO5, and Sm0.2Gd0.2Dy0.2Yb1.4TiO5. The aim was to produce single‐phase novel materials with cubic symmetry, capable of incorporating a wide variety of cations and with acceptable radiation tolerance. The chemistry flexibility and radiation tolerance are some of the major desirable properties for nuclear waste‐form materials. By using multiple lanthanides the average lanthanide radius can be controlled and consequently the structure, along with properties such as radiation tolerance. The radiation tolerance was assessed using in situ 1 MeV krypton irradiation and transmission electron microscopy characterization. Those materials for which cubic symmetry was achieved displayed better radiation tolerance; a greater critical fluence of ions (Fc) was required for the crystalline to amorphous transition, and a lower temperature was required to maintain crystallinity (Tc) during irradiation. [ABSTRACT FROM AUTHOR]

We refine the crystal structures of a systematic series of compounds with the general composition Ho (x) Yb (2-x) TiO 5 (x = 2, 1.6, 1.2, 1, 0.8, 0.4, 0) and Er 2 TiO 5 and find a transition from defect-pyrochlore to defect-fluorite structure with increasing ytterbium content, decreasing lanthanide radius. Short-range structure modulations consisting of pyrochlore-like nano-domains are systematically characterised using transmission electron microscopy. We test the Kr 2+ 1 MeV ion-irradiation response of Ho 2 TiO 5 , HoYbTiO 5 , Yb 2 TiO 5 , and Er 2 TiO 5 , via the crystalline to amorphous transition observed by using the in-situ TEM approach. The critical dose of amorphisation, D c , was measured at various temperatures and used to calculate the critical temperature for maintaining crystallinity, T c . A trend of lower T c values with decreasing lanthanide radius is found. We describe a new approach for determining T c values using cross-sectional TEM analysis of ex-situ bulk irradiated, 1 MeV Se + , samples; Ho 2 TiO 5 , HoYbTiO 5 and Yb 2 TiO 5 . The results of D c and T c values using the two approaches vary; however the trends across the sample system remain the same. [ABSTRACT FROM AUTHOR]

Abstract: The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation–anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation–anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature. [Copyright &y& Elsevier]

In the search for novel ceramics for use within nuclear fuel–related applications and nuclear waste-form matrices, a major focus has been on the development of radiation-tolerant materials. Of particular interest in this field have been numerous compounds with either pyrochlore or the related fluorite-type structures. In this study, we look to expand the family of compounds with defect fluorite–type structure. We have fabricated three new compounds; Yb2Sn1.125O5.25, Yb2Sn1.25O5.5, and Yb2Sn1.375O5.75. The compound Yb2Sn1.125O5.25 was determined, via x-ray diffraction, to have the long-range defect fluorite structure, Fm-3 m symmetry, with cell parameter a = 5.17233(1). Further to this, Sm2ZrO5 and Yb2TiO5 compounds were also fabricated and crystal structures characterised. The use of transmission electron microscopy has revealed a much more complex crystal structure than that of the relatively high symmetry fluorite, with the presence of structural modulations being detected. The ion-irradiation response of these compounds was tested via bulk specimen irradiation using 15-MeV gold ions with grazing incidence x-ray diffraction characterisation. The results show that both Sm2ZrO5 and Yb2Sn1.25O5.5 are highly tolerant to ion-irradiation exposure whilst Yb2TiO5 is susceptible to amorphisation. [ABSTRACT FROM AUTHOR]

Abstract: Time-of-flight powder neutron diffraction has been performed on oxides with composition (La1− x Nd x )2Zr2O7 and Nd2(Zr1− x Ti x )2O7, where , 0.2, 0.4,…1.0, in order to determine the solid solution behaviour across each series. Between La2Zr2O7 and Nd2Zr2O7, a cubic pyrochlore phase is observed (, ). A linear decrease in the lattice parameter from 10.8047 to 10.6758Å indicates complete miscibility of the two end-members. For the same series, the oxygen x-parameter increases from 0.3313 to 0.3348, suggesting increased distortion of the 6 coordinate B sites and reduced distortion of the 8 coordinate sites. There is limited solubility of Nd2Ti2O7 in Nd2Zr2O7. Exsolution of a monoclinic phase (, ) rich in Nd2Ti2O7 is observed at approximately . The compositional range over which a solid solution exists is more extensive than that which has been previously reported. The solubility of Nd2Zr2O7 in Nd2Ti2O7 is very low. [Copyright &y& Elsevier]

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]

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]

The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. [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]

Abstract: The incorporation of plutonium (Pu) within lanthanum zirconate pyrochlore was investigated using air, argon, and N2–3.5%H2 sintering atmospheres together with Ca2+ and Sr2+ incorporation for charge compensation. The samples have been characterised in the first instance by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results show Pu can be exchanged for La3+ on the A-site with and without charge compensation and for Zr4+ on the B-site. DRS measurements were made over the wavenumber range of 4000–19,000cm−1 and the Pu in all air- and argon-sintered samples was found to be present as Pu4+ while that in samples sintered in N2–3.5%H2 was present as Pu3+. The Pu valence was confirmed for three of the samples using X-ray near-edge absorption spectroscopy (XANES). Pu valences >4+ were not observed in any of the samples. [Copyright &y& Elsevier]

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]

Abstract: Pyrochlores based on the general composition Ln2TiO5 (Ln2.67Ti1.33O6.67) and Ln2Ti2O7, where Ln=Y or Yb, have been irradiated through the crystalline–amorphous transition with 1MeV Kr ions at the IVEM-TANDEM facility, Argonne National Laboratory. The results show that the Tc (critical temperature for amorphization) differs significantly between each series, e.g. for Y2TiO5 it is 589±18K and for Y2Ti2O7 665±33K. The difference suggests that recovery from damage is more rapid with increasing Ln content, i.e. a lower Tc for amorphization. These results are discussed in the context of the melting points of each phase, atomic disorder, the pyrochlore–fluorite order–disorder transition and the implications for oxide dispersion-strengthened additives. [Copyright &y& Elsevier]

Abstract: The crystal structures of the compounds La2− x Y x Zr2O7 and La2− x Y x Hf2O7 with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La0.8Y1.2Zr2O7 and La0.4Y1.6Hf2O7, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases. [Copyright &y& Elsevier]