Browsing by Author "Carter, ML"
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- ItemAb initio structure determinations of high-performance radioactive waste forms(Elsevier B. V., 2006-11-15) Carter, ML; Wallwork, KS; James, M; Withers, RLImmobilisation of high-level radioactive wastes in stable matrices for long-term storage or geological disposal is a key step in closing the nuclear fuel cycle. Titanate minerals have shown much promise in their capacity to take up radioactive elements into their crystal structures at regular lattice sites in pyrochlore and brannerite phases. Previously unreported uranyl titanates BaUTiO6 and Na2U3Ti2O11 have been synthesised under argon at 1400 and 1250 deg. C, respectively and their structures determined by ab initio techniques using a combination of electron, synchrotron and neutron powder diffraction. BaUTiO6 crystallises with a carnotite-type structure (P21/c; a=6.4463(1) A, b=8.5999(1) A, c=10.2532(1) A; >=75.936(1)o and V=551.36(1) A3); while Na2U3Ti2O11 forms an orthorhombic structure (Pnma; a=31.0421(8) A, b=7.2774(2) A, c=7.6111(2) A and V=1719.4(1) A3). Due to their potential value as a radioactive waste forms, the leach rates of uranium, barium and sodium were determined. Normalised uranium leach rates from BaUTiO6 and Na2U3Ti2O11, averaged over 7 days were orders of magnitude less than those for brannerite and for U-containing pyrochlore waste forms. Crown Copyright © 2006 Published by Elsevier B.V
- ItemAb initio structure determinations of high-performance radioactive waste forms(The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Carter, ML; Wallwork, KS; James, M; Withers, RLImmobilisation of high-level radioactive wastes in stable matrices for long-term storage or geological disposal is a key step in closing the nuclear fuel cycle. Titanate minerals have shown much promise in their capacity to take up radioactive elements into their crystal structures at regular lattice sites in pyrochlore and brannerite phases. Previously unreported uranyl titanates BaUTiO6 and Na2U3Ti2O11 have been synthesised under argon at 1400 and 1250 deg. C, respectively and their structures determined by ab initio techniques using a combination of electron, synchrotron and neutron powder diffraction. BaUTiO6 crystallises with a carnotite-type structure (P21/c; a=6.4463(1) A, b=8.5999(1) A, c=10.2532(1) A; >=75.936(1)o and V=551.36(1) A3); while Na2U3Ti2O11 forms an orthorhombic structure (Pnma; a=31.0421(8) A, b=7.2774(2) A, c=7.6111(2) A and V=1719.4(1) A3). Due to their potential value as a radioactive waste forms, the leach rates of uranium, barium and sodium were determined. Normalised uranium leach rates from BaUTiO6 and Na2U3Ti2O11, averaged over 7 days were orders of magnitude less than those for brannerite and for U-containing pyrochlore waste forms. © The Authors
- ItemAntiferromagnetism in a technetium oxide. Structure of CaTcO3(American Chemical Society, 2011-01-16) Avdeev, M; Thorogood, GJ; Carter, ML; Kennedy, BJ; Ting, J; Singh, DJ; Wallwork, KSThe technetium perovskite CaTcO3 has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO3 is an antiferromagnetic with a surprisingly high Neel temperature of ~800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results. © 2011, American Chemical Society
- ItemCrystal chemistry and structures of (Ca,U) titanate pyrochlores(Wiley-Blackwell, 2010-10-01) James, M; Carter, ML; Zhang, ZM; Zhang, YJ; Wallwork, KS; Avdeev, M; Vance, ERAnalysis of multiphase (Ca,U) titanate pyrochlore samples produced under argon or in air, using laboratory X-ray powder diffraction (XRD) and electron microscopy revealed discrete pyrochlore phase compositions, together with either perovskite or rutile impurities. Investigations of these samples using diffuse reflectance and X-ray photoelectron spectroscopies revealed mixed U4+/U5+ oxidation states for argon-annealed samples and U5+/U6+ oxidation states for air-annealed samples. Single-phase (Ca1.25U4+0.25U5+0.50)Ti2O7 (argon) and (Ca1.40U5+0.60)(Ti1.90U6+0.10)O7 (air) compositions were synthesized and characterized using a combination of synchrotron X-ray and neutron powder diffraction. Severe intensity reduction of hkl odd XRD peaks from the sample produced in air was found to be the result of U6+ sharing the Ti site. The refined crystal structures for both single-phase samples were found to be consistent with the compositions and mixed oxidation states observed by the above spectroscopic measurements. Although the normalized uranium leach rate of the argon-annealed sample was found to be approximately twice that of the air-annealed sample, in 7-day Product Consistency Test the durability of both compounds with respect to leaching by water was found to be excellent. Ca releases were in the range of 0.01–0.03 g/L and U releases were <3 × 10−5 g/L. © 2010, Wiley-Blackwell.
- ItemHigh temperature magnetic ordering in the 4d perovskite SrTcO3(American Physical Society, 2011-02-07) Rodriguez, EE; Poineau, F; Llobet, A; Kennedy, BJ; Avdeev, M; Thorogood, GJ; Carter, ML; Seshadri, R; Singh, DJ; Cheetham, AKWe present evidence for possibly the highest magnetic ordering temperature in any compound without 3d transition elements. Neutron powder diffraction measurements, at both time-of-flight and constant wavelength sources, were performed on two independently prepared SrTcO3 powders. SrTcO3 adopts a distorted perovskite structure with G-type antiferromagnetic ordering and has a moment of 1.87(4)μB per Tc cation at room temperature with an extraordinarily high Néel point close to 750°C. Electronic structure calculations reveal extensive mixing between the technetium 4d states and oxygen states proximal to the Fermi level. This hybridization leads to a close relationship between magnetic ordering temperature and moment formation in SrTcO3. © 2011, American Physical Society
- ItemHIPed tailored hollandite waste forms for the immobilization of radioactive Cs and Sr.(Wiley-Blackwell, 2009-05) Carter, ML; Gillen, AL; Olufson, KP; Vance, ERHot isostatically pressed tailored hollandite waste forms were used to demonstrate the immobilization of Cs and Sr are separable from spent nuclear fuel, as well as Ba and Rb. Four hollandite formulations were investigated, two samples with ∼12 wt% waste loading (on an oxide basis) and two with ∼18 wt% waste loading. Two of the samples were Al-substituted and the other two contained Mg. The hollandite in the Al-substituted samples contained all the waste cations, as designed, but this was not the case in the Mg-substituted samples. The hollandite in the Mg-substituted samples did not contain all the waste cations, with ∼50% of the Sr forming SrTiO3 as a secondary phase. This resulted in waste forms that were not as durable, with respect to Cs, as their Al counterparts. The formation of SrTiO3 had little effect on the Sr release rates and was not detrimental to the Mg-substituted hollandite waste form. For the Al-substituted samples, the MCC-1 normalized release rates were <0.06 g·(m2·day)−1 at 0–28 days for all elements, while the Cs release rates remained at 2.0 g·(m2·day)−1 at 0–28 days for the Mg-substituted samples. © 2009, Wiley-Blackwell
- ItemIncorporation of uranium in zirconolite (CaZrTi2O7)(John Wiley & Sons, Inc, 2002-07) Vance, ER; Lumpkin, GR; Carter, ML; Cassidy, DJ; Ball, CJ; Day, RA; Begg, BDUp to 0.15 formula unit (f.u.) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1−x)Ti2O7 compositions in argon at 1400°C allows retention of the 2M polytype. Further U4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing ∼0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid-state reactivity of even alkoxide-based preparations becomes increasingly poor for x > ∼0.7 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single-phase pyrochlore structures for x= 1. When samples of CaUxZr1−xTi2O7 stoichiometry (x= 0.1 and 0.2) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, ∼50 nm in half-width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon-fired samples with x= 0.1 and 0.4 are heated in air to 1200°C. Evidence for U valence states higher than +4 in both argon- and air-heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES. DRS shows weak absorption bands attributable to U4+ in zirconolites containing ∼0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace-cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials. © 2002 The American Ceramic Society
- ItemMicrostructural and spectroscopic investigations into the effect of CeO2 additions on the performance of a MnO2 aqueous rechargeable battery(Elsevier, 2009-04-30) Minakshi, M; Mitchell, DRG; Carter, ML; Appadoo, D; Nallathamby, KThe influence of CeO2 additions on the electrochemical behaviour of the MnO2 cathode in a Zn–MnO2 battery using lithium hydroxide (LiOH) as an electrolyte is investigated using microscopy and spectroscopic techniques. The results showed that such additions greatly improve the discharge capacity of the battery (from 155 to 190 mAh g−1) but only from the second discharge cycle onwards. Capacity fade with subsequent cycling is also greatly reduced. With an aim to understand the role of CeO2 on the discharge–charge characteristics of MnO2 and its mechanism, we have used a range of microscopy, spectroscopy and diffraction-based techniques to study the process. The CeO2 is not modified by multiple discharged and charged cycles. The CeO2 may enhance the discharge–charge performance of the battery by raising the oxygen evolution potential during charging but does not take part directly in the redox reaction. © 2009, Elsevier Ltd.
- ItemOxygen deficient lead-technetium pyrochlore, the first example of a stable valence V technetium oxide?(European Association of Geochemistry and the Geochemical Society (Goldschmidt), 2021-07-05) Thorogood, GJ; Avdeev, M; Carter, ML; Losurdo, L; Saura-Múzquiz, M; Thorogood, KJ; Ting, J; Wallwork, KS; Zhang, Z; Kennedy, BJDespite the fact that Technetium V oxides are possible there are very few reports of their existence. Most recently Lawler et.al. [1] have reported the structure of Tc2O5 “tech red” and have noted that it is indeed volatile. It is apparent from this study that there is no stable form and they draw parallels with a well-studied analogue of Tc2O5, Re2O5 that disprortioniates into Re(4+) and Re(7+) species. Given these parallels we investigated PbTcO3 as reported by Muller et.al [2] to be a pyrochlore in an attempt to determine if there were parallels with Pb2Re2O7-d. The structure of lead-technetium pyrochlore has been refined in space group with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O6.86. The displacive disorder of the Pb cations is evident from the refinements as has been observed Bi2Tc2O7-d. X-ray absorption measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the local coordination of the Technetium leading us to conclude that this pyrochlore is the first example of a stable valence V Technetium oxide. [1] Lawler, K. V. et al. Unraveling the mystery of ‘tech red’-a volatile technetium oxide. Chem. Commun. 54, 1261–1264 (2018). [2] Muller, O., White, W. B. & Roy, R. Crystal chemistry of some technetium-containing oxides. J. Inorg. Nucl. Chem. 26, 2075–2086 (1964).
- ItemPu, U, and Hf incorporation in Gd silicate apatite(John Wiley & Sons, Inc, 2004-12-20) Vance, ER; Ball, CJ; Begg, BD; Carter, ML; Day, RA; Thorogood, GJTrivalent Pu can be incorporated in the silicate apatite structure to form Ca2Pu8(SiO4)6O2 by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d-spacings and intensities of the X-ray pattern of hexagonally structured Ca2Pu8(SiO4)6O2 after firing at 1250°C are given, and the a and c lattice parameters are 0.95611 and 0.70281 nm, respectively. The respective solid solubility limits of U and Hf in Ca2Gd8–x(U/Hf)x(SiO4)6O2 apatite samples were 0.3 and 0.2 f.u. © 2004 The American Ceramic Society
- ItemSolid solubilities of Pu, U, Gd and Hf in candidate ceramic nuclear wasteforms(Australian Nuclear Science and Technology Organisation, 2001-04-02) Vance, ER; Carter, ML; Lumpkin, GR; Day, RA; Begg, BDThe goal of this research project was to determine the solid solubility of Pu, U, Gd, and Hf in candidate ceramics for immobilization of high-level nuclear waste. The experimental approach was to saturate each phase by adding more than the solid solubility limit of the given cation, using a nominated substitution scheme, and then analyzing the candidate phase that formed to evaluate the solid solubility limit under firing conditions. Confirmation that the solid solution limit had been reached insofar as other phases rich in the cation of interest was also required. The candidate phases were monazite, titanite, zirconolite, perovskite, apatite, pyrochlore, and brannerite. The valences of Pu and U were typically deduced from the firing atmosphere, and charge balancing in the candidate phase composition as evaluated from electron microscopy, although in some cases it was measured directly by x-ray absorption and diffuse reflectance spectroscopies (for U). Tetravalent Pu and U have restricted (< 0.1 formula units) solid solubility in apatite, titanite, and perovskite. Trivalent Pu has a larger solubility in apatite and perovskite than Pu4+. U3+ appears to be a credible species in reduced perovskite with a solubility of {approximately} 0.25 f.u. as opposed to {approximately} 0.05 f.u. for U4+. Pu4+ is a viable species in monazite and is promoted at lower firing temperatures ({approximately} 800 C) in an air atmosphere. Hf solubility is restricted in apatite, monazite (< 0.1 f.u.), but is {approximately} 0.2 and 0.5 f.u. in brannerite and titanite, respectively. Gd solubility is extended in all phases except for titanite ({approximately} 0.3 f.u.). U5+ was identified by DRS observations of absorption bands in the visible/near infrared photon energy ranges in brannerite and zirconolite, and U4+ in zirconolite was similarly identified.
- ItemStructural phase transitions and magnetic order in SrTcO3(Royal Society of Chemistry, 2011-07-21) Thorogood, GJ; Avdeev, M; Carter, ML; Kennedy, BJ; Ting, J; Wallwork, KSThe structure of the perovskite SrTcO3 has been investigated using both synchrotron X-ray and neutron powder diffraction. At room temperature SrTcO3 is orthorhombic as a consequence of cooperative tilting of the corner sharing TcO6 octahedra. The tilts are sequentially removed as the sample is heated with the oxide displaying the sequence of structres Pnma [rightward arrow] Imma [rightward arrow] I4/mcm [rightward arrow] Pm3[3 with combining macron]m. Neutron powder diffraction data collected in the temperature range 4-1023 K indicate that SrTcO3 has G-type antiferromagnetic structure, in which each Tc moment is antiparallel to its six nearest neighbours, below [similar]1000 K. The magnetic structure is collinear antiferromagnetic with the technetium moments parallel to c-axis and can be described by the propagation vector k = [0,0,0] and the basis vector (0,0,Az). The same magnetic structure is observed in each of the four crystal structures.© 2011, Royal Society of Chemistry
- ItemStructural studies of SrTcO3, CaTcO3, and Pb2Tc2O7-d(International Union of Crystallography, 2011-08-22) Thorogood, GJ; Kennedy, BJ; Avdeev, M; Ting, J; Zhang, Z; Carter, MLLittle is known regarding the structure and properties of technetium oxides. This is mainly due to the absence of any naturally occurring Tc isotopes and the radioactivity of those available [1]. Tc oxides are important since 99Tc is found in nuclear waste and has a long half-life (2.13 × 105 yr). Tc is a 4d element, and like its lighter analogue Mn a 3d element it can adopt a number of oxidation states. 3d oxides are generally, but not always magnetic, while 4d and 5d oxides, with their more spatially extended d-orbitals tend not to be magnetic although SrRuO3 [2] is a notable exception. Here we have combined synchrotron X-ray and neutron diffraction to examine the structure of CaTcO3, SrTcO3 and Pb2Tc2O7-d. The only previous report on these materials was by Muller, White and Roy in 1964 [3]. They reported the existence of these materials and suggested the CaTcO3 and SrTcO3 were orthorhombic whereas the Pb2Tc2O7-d was reported as being pyrochlore. This earlier work concentrated on syn-thesis hence little is known about these three compounds and there has been no study of their structures or magnetic properties. © International Union of Crystallography
- ItemSynroc derivatives for the Hanford waste remediation task(Materials Research Society, 1997) Vance, ER; Hart, KP; Day, RA; Carter, ML; Hambley, M; Blackford, MG; Begg, BDThree wt% each of Cs and Tc were mixed with the standard Synroc precursor and the ceramic was formed by hot-pressing. Attempts were made to incorporate the Tc as either metal or Tc 4+, using different redox conditions in processing. Volatile losses of Tc during calcination were < 0.1% in all cases. Short-term Tc leach rates when the Tc was present as a metal alloy were in the order of 10−4 g/m /d at 90°C with frequently changed water, and decreased with increasing leaching time. The valence of the Tc was monitored by X-ray absorption spectroscopy at the drying and calcination stages of the production. The general viability of Synroc/glass composites for immobilising the Hanford HLW sludges is further demonstrated by using further refinements of additive schemes for the inactive “All-blend” formulation and initial studies using the U-containing “All-blend” waste formulation. © Materials Research Society 1997
- ItemSynthesis and structure of oxygen deficient lead-technetium pyrochlore, the first example of a valence V technetium oxide(Frontiers Media, 2021-07-01) Kennedy, BJ; Ablott, TA; Avdeev, M; Carter, ML; Losurdo, L; Saura-Múzquiz, M; Thorogood, KJ; Ting, J; Wallwork, KS; Zhang, ZM; Zhu, HL; Thorogood, GJThe structure of lead-technetium pyrochlore has been refined in space group F d 3 ¯ m with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O7-d. Displacive disorder of the Pb cations is evident from the refinements, as has been observed in Bi2Tc2O7-d. X-ray absorption spectroscopic measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the presence of disorder leading us to conclude that this pyrochlore is the first example of a valence V technetium oxide. © 2021 Kennedy, Ablott, Avdeev, Carter, Losurdo, Saura-Muzquiz, Thorogood, Ting, Wallwork, Zhang, Zhu and Thorogood. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY).
- ItemTitanate ceramics for immobilisation of uranium-rich radioactive wastes arising from Mo-99 production(Elsevier, 2009-02-28) Carter, ML; Li, HJ; Zhang, YJ; Vance, ER; Mitchell, DRGUranium-rich liquid wastes arising from UO2 targets which have been neutron-irradiated to generate medical radioisotopes such as 99mTc require immobilisation. A pyrochlore-rich hot isostatically pressed titanate ceramic can accommodate at least 40 wt% of such waste expressed on an oxide basis. In this paper, the baseline waste form composition (containing 40 wt% UO2) was adjusted in two ways: (a) varying the UO2 loading with constant precursor oxide materials, (b) varying the precursor composition with constant waste loading of UO2. This resulted in the samples having a similar phase assemblage but the amounts of each phase varied. The oxidation states of U in selected samples were determined using diffuse reflection spectroscopy (DRS) and electron energy loss spectroscopy (EELS). Leaching studies showed that there was no significant difference in the normalised elemental release rates and the normalised release rates are comparable with those from synroc-C. This demonstrates that waste forms based on titanate ceramics are robust and flexible for the immobilisation of U-rich waste streams from radioisotope processing. © 2009, Elsevier Ltd.
- ItemAn x-ray photoelectron spectroscopy investigation of highly soluble grain-boundary impurity films in hollandite(Wiley-Blackwell, 2010-03) Zhang, Z; Carter, MLX-ray photoelectron spectroscopy was used to detect an intergranular impurity film in two (BaxCsy)(MnzTi8−z)O16 hollandite samples sintered in argon and air, respectively. This impurity film is enriched in Cs and Ba, as a result of segregation during high-temperature processing. The Cs ions in the grain-boundary layer have a different chemical bonding state from that in the hollandite phase, as evidenced by a large chemical shift of the Cs 3d and Cs 4d levels. The chemical environment around the Ba ions in the intergranular film is also likely to be different from that in the hollandite. The presence of this grain-boundary film is responsible for the relatively high initial loss of Cs and Ba during the aqueous dissolution testing. © 2010, Wiley-Blackwell.
- ItemZirconolite-rich titanate ceramics for immobilisation of actinides - waste form/HIP can interactions and chemical durability(Elsevier, 2009-12) Zhang, YJ; Stewart, MWA; Li, HJ; Carter, ML; Vance, ER; Moricca, SAZirconolite-based titanate ceramics containing U plus Th or Pu have been prepared. The final consolidation to produce a dense monolithic waste form was carried out using hot isostatic pressing (HIPing) of the calcined materials within a stainless steel can. The ceramics were characterised and tested for their overall feasibility to immobilise impure Pu or separated actinide-rich radioactive wastes. As designed, tetravalent U and Pu are mainly incorporated in a durable zirconolite phase, together with Gd or Hf added as neutron absorbers. The interaction of the waste form with the HIP can was also examined. No changes in the U valences or the U/Pu-bearing phase distributions were observed at the waste form–HIP can interface. © 2009, Elsevier Ltd.