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Browsing by Author "Murphy, GL"

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Controlling oxygen defect formation and its effect on reversible symmetry lowering and disorder-to-order phase transformations in nonstoichiometric ternary uranium oxides
(American Chemical Society, 2019-04-09) Murphy, GL; Wang, CH; Zhang, Z; Kowalski, PM; Beridze, G; Avdeev, M; Muránsky, O; Brand, HEA; Gu, QF; Kennedy, BJ
In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4–x (A = alkaline earth metal cation) oxides CaUO4–x and α-Sr0.4Ca0.6UO4–x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅m) CaUO4–x and α-Sr0.4Ca0.6UO4–x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4–x and α-Sr0.4Ca0.6UO4–x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4–x and α-Sr0.4Ca0.6UO4–x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect–defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect–defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4–x and Sr0.4Ca0.6UO4–x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δ-SrUO4–x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature. © 2019 American Chemical Society
Ion-transport phenomena and anomalous transformations in strontium uranium oxides.
(International Union of Crystallography, 2017-12-01) Murphy, GL; Zhang, Z; Avdeev, M; Wang, CH; Beridze, G; Kowalski, PM; Gu, QF; Kimpton, JA; Johannessen, B; Kennedy, BJ
Structural-chemical elucidation of low dimensional ternary uranium oxide systems is considered an essential aspect of thenuclear fuel cycle since understanding of their physicochemical properties may guide the storage and disposal of spentnuclear fuel [1]. The study of these systems allows for further exploration of the peculiar, exotic and poorly knownproperties of materials containing, or which can access, 5f electrons. SrUO₄ exemplifies this, a potential waste form resultingfrom reaction between spent UO₂+x fuel and the fission daughter Sr-90. We have found, through a combination of in situsynchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that during its first order rhombohedral-orthorhombic transition under oxidising conditions, the rhombohedral form of SrUO₄, α, undergoes a spontaneousreduction of the uranium valence state through oxygen vacancy formation [2]. The process is synergetic, as the triality ofoxygen vacancy formation, subsequent ion diffusion and uranium reduction, seemingly reduces the activation energy barrierfor the transformation to the thermodynamically favoured stoichiometric orthorhombic form, β-SrUO₄. However formation ofthe orthorhombic form is only possible if a source of oxygen is present, without this, the oxygen deficient α-SrUO₄-xremains rhombohedral as shown by in situ neutron powder diffraction measurements. These experimental observations arefurther supported by ab initio DFT+U calculations using the self consistently calculated Hubbard U parameter values andbond valence sums calculations [2-3]. These methods indicate the affinity for α-SrUO₄-x to retain oxygen vacancies asopposed to β-SrUO₄, a consequence of the crystal lattice’s ability to stabilise the coordination environment of the Sr²⁺ cationvia the flexibility of uranium to undergo reduction through vacancy formation. CaUO4, isostructural to α-SrUO₄ , but unlike α-SrUO₄ does not have a stable orthorhombic polymorph as shown by both insitu synchrotron X-ray powder diffraction measurements and ab initio calculations. Introducing Sr ions into the CaUO₄ latticein the form of a solid solution, α-Sr₁-xCaxUO₄ (0 < x < 0.4), provides a means to atomically engineer the lattice to promoteoxygen vacancy formation, and presumably diffusion, at high temperatures. When CaUO₄ or α-SrUO₄ is treated underhighly reducing conditions, both materials undergo unusual reconstructive phase transformations at high temperatures to amonoclinic structure. These phase transformations are reversible, and cooling the sample yields the correspondingrhombohedral structure again. It is remarkable that the ordered monoclinic structure is favoured at high temperatures andthe disordered rhombohedral structure at low temperatures. This investigation in SrUO₄ highlights the rich and remarkablestructural chemistry and crystallography that may be found within poorly understood actinide systems whilst demonstratingthe successful marriage of experimental and theoretical approaches towards elucidating their chemical and physicalphenomena. © International Union of Crystallography
Nonstoichiometry in strontium uranium oxide: understanding the rhombohedral–orthorhombic transition in SrUO4
(American Chemical Society, 2016-08-29) Murphy, GL; Kennedy, BJ; Kimpton, JA; Gu, QF; Johannessen, B; Beridze, G; Kowalski, PM; Bosbach, D; Avdeev, M; Zhang, Z
In situ neutron and synchrotron X-ray diffraction studies demonstrate that SrUO4 acts as an oxygen transfer agent, forming oxygen vacancies under both oxidizing and reducing conditions. Two polymorphs of SrUO4 are stable at room temperature, and the transformation between these is observed to be associated with thermally regulated diffusion of oxygen ions, with partial reduction of the U6+ playing a role in both the formation of oxygen deficient α-SrUO4−δ and its subsequent transformation to stoichiometric β-SrUO4. This is supported by ab initio calculations using density functional theory calculations. The oxygen vacancies play a critical role in the first order transition that SrUO4 undergoes near 830 °C. The changes in the oxidation states and U geometry associated with the structural phase transition have been characterized using X-ray absorption spectroscopy, synchrotron X-ray diffraction, and neutron diffraction. © 2016 American Chemical Society
Phase analysis of Australian uranium ore concentrates determined by variable temperature synchrotron powder x-ray diffraction
(American Chemical Society, 2021-07-22) Pandelus, SB; Kennedy, BJ; Murphy, GL; Brand, HEA; Keegan, EA; Pring, A; Popelka-Filcoff, RS
The chemical speciation of uranium oxides is sensitive to the provenance of the samples and their storage conditions. Here, we use diffraction methods to characterize the phases found in three aged (>10 years) uranium ore concentrates of different origins as well as in situ analysis of the thermally induced structural transitions of these materials. The structures of the crystalline phases found in the three samples have been refined, using high-resolution synchrotron X-ray diffraction data. Rietveld analysis of the samples from the Olympic Dam and Ranger uranium mines has revealed the presence of crystalline α-UO2(OH)2, together with metaschoepite (UO2)4O(OH)6·5H2O, in the aged U3O8 samples, and it is speculated that this forms as a consequence of the corrosion of U3O8 in the presence of metaschoepite. The third sample, from the Beverley uranium mine, contains the peroxide [UO2(η2-O2)(H2O)2] (metastudtite) together with α-UO2(OH)2 and metaschoepite. A core–shell model is proposed to account for the broadening of the diffraction peaks of the U3O8 evident in the samples. © 2021 American Chemical Society
Radiation-induced micro-structures as ground states of a Swift-Hohenberg energy functional
(American Institute of Physics (AIP), 2019-02-08) Simeone, D; Thorogood, GJ; Murphy, GL; Forestier, A; Garcia, P; Luneville, L
We demonstrate that the Swift-Hohenberg functional, which is used to describe patterning observed in out of equilibrium systems such as diblock copolymers, Rayleigh-Benard convection, and thin film magnetic garnets, can be applied to radiation-induced patterns that occur in non-miscible alloys. By comparing ground states obtained from the minimization of this functional and a 2D numerical simulation performed on an irradiated AgCu material, which is the archetype of a non-miscible alloy, we show that the Swift-Hohenberg functional provides all possible patterns generated under irradiation and the solubility limits of radiation-induced precipitates in these patterns. To rationalize the formation of these radiation-induced patterns, we propose a generic “pseudophase diagram” that relies not only on the irradiation flux and temperature but also on the overall composition of the alloy. Tuning this overall composition offers the opportunity to tailor new materials with various micro-structures overcoming the limitation of the equilibrium phase diagram. © 2019 Author(s). Published under license by AIP Publishing.
Structural studies of the rhombohedral and orthorhombic monouranates: CaUO4, α-SrUO4, β-SrUO4 and BaUO4
(Elsevier B.V., 2016-05-01) Murphy, GL; Kennedy, BJ; Johannessen, B; Kimpton, JA; Avdeev, M; Griffith, CS; Thorogood, GJ; Zhang, Z
The structures of some AUO4 (A=Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L3-edge. The smaller Ca cation favours a rhombohedral AUO4 structure with 8-coordinate UO8 moieties whilst an orthorhombic structure based on UO6 groups is found for BaUO4. Both the rhombohedral and orthorhombic structures can be stabilised for SrUO4. The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO4, is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. © 2016 Elsevier Inc.
Structural transformative behaviour in barium thorium oxides: a neutron and synchrotron x-ray diffraction study
(Australian Institute of Nuclear Science and Engineering, 2016-11-30) Murphy, GL; Kennedy, BJ; Avdeev, M; Zhang, Z
The increasing interest in nuclear energy, as a response to demands for a transition to non-fossil fuel energy sources, has reinvigorated attempts to explore a thorium fuel cycle. Thorium is seen as a more attractive option to uranium as a nuclear fuel, due to its enhanced proliferation resistance and reduced transuranic generation. Thorium reactors face several challenges particularly related to the large paucity of information of fuel fission by product material phases. A pertinent structure in this regard is BaThO3, due its potential to host the fission daughter Ba-137m. The radioactivity of thorium coupled with the unusual decomposition behaviour of BaThO3, has prevented detailed analysis of the material. The present study has utilized a combination of neutron and synchrotron x-ray diffraction in order to establish the structure of BaThO_3 and elucidate its high temperature structural behaviour. Our results indicate BaThO_3 undergoes a reversible second order continuous phasetransformation at 975 K from the space group Pbnm to Imma. Neutron and synchrotron Xrays were key to this, successfully resolving the characteristic superlattice X-point and Mpoint reflections present in Pbnm and absent in Imma. Considering the decomposition behaviour of BaThO_3 attempts were made to include Th(IV) partially on the B site of otherABO_3 perovskites including BaZrO_3 and SrZrO_3 through the formation of appropriate solid solutions. This resulted in maximum Th solubilities of less than 10 %. Surprisingly greater solubilities are known for Th(IV) on the A site of in layered perovskites such as Na_2_/_3Th_1_/_3TiO_3. The ability of Th to occupy the perovskite A-site is in contrast to that what is typically observed for uranium and other nuclear fuel relevant perovskites.
Structure and phase transition in BaThO3: a combined neutron and synchrotron x-ray diffraction study
(Elsevier, 2017-12-15) Murphy, GL; Kennedy, BJ; Zhang, Z; Avdeev, M; Brand, HEA; Kegler, P; Alekseev, EV
The structure of BaThO3, obtained by solid state synthesis, was refined for the first time by the Rietveld method using a combination of synchrotron X-ray and neutron powder diffraction data. BaThO3 has an orthorhombic structure at room temperature, in space group Pbnm with a = 6.3491(5), b = 6.3796(4) and c = 8.9907(7) Å. Heating BaThO3 to above 700 °C results in a continuous transition to a second orthorhombic structure, in space group Ibmm, demonstrated by both in situ neutron and synchrotron X-ray powder diffraction measurements. The coefficient of volumetric thermal expansion for BaThO3 is determined to be 1.04 × 10−5 °C-1 from 50 to 625 °C (Pbnm phase), and 9.43 × 10−6 °C-1 from 800 to 1000 °C (Ibmm phase). BaThO3 was found to decompose upon exposure to atmospheric moisture resulting in the formation of ThO2. The thermal expansion of ThO2, which invariably co-exists with BaThO3, is also described. © 2017 Elsevier B.V.
Studies of the antiferrodistortive transition in EuTiO3
(Institute of Physics, 2014-11-12) Kennedy, BJ; Murphy, GL; Reynolds, EM; Avdeev, M; Brand, HEA; Kolodiazhnyi, T
Structural studies of EuTiO3, conducted using synchrotron x-ray powder diffraction, reveal the sensitivity of this material to defects. The presence of a single tetragonal–cubic (I4/mcm–Pm-3m) transition is confirmed. Neutron diffraction measurements show EuTiO3 to have a G-type antiferromagnetic arrangement. Examinations of the symmetry-adapted tetragonal strains demonstrate that the stoichiometry impacts on the continuous nature of this. The impact of defects on the electrical conductivity and magnetodielectric effect is also described. © 2014 IOP Publishing Ltd
Tilting and distortion in rutile-related mixed metal ternary uranium oxides: a structural, spectroscopic, and theoretical investigation
(American Chemical Society, 2021-01-29) Murphy, GL; Zhang, Z; Tesch, R; Kowalski, PM; Avdeev, M; Kuo, EY; Gregg, DJ; Kegler, P; Alekseev, EV; Kennedy, BJ
A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO4 [Murphy et al. Inorg. Chem.2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides. © 2021 American Chemical Society
Unexpected crystallographic phase transformation in nonstoichiometric SrUO4–x: Reversible oxygen defect ordering and symmetry lowering with increasing temperature
(American Chemical Society, 2018-05-01) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Kimpton, JA; Avdeev, M; Kowalski, PM; Kennedy, BJ
In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4–x, in space group R3̅m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4–x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4–x. Cooling δ-SrUO4–x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4–x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4–x to enable the transformation to δ-SrUO4–x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4–x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4–x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4–x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations. © 2018 American Chemical Society
Unique capabilities and methods for analysing nuclear waste form materials using ANSTO’s landmark infrastructure
(Materials Research Society (MRS), 2017-10-29) Murphy, GL; Kennedy, BJ; Brand, HEA; Maynard-Casely, HE; Avdeev, M; Zhang, Z
In situ investigations of nuclear waste form related materials under extreme conditions are highly desirable for understanding and optimising applications of these materials, however they are often exceedingly challenging from a safety and radiological perspective. As a part of our investigations into actinide materials, we have developed a number of novel experimental methodologies which utilize ANSTO’s landmark research infrastructure, the Opal nuclear reactor and Australian Synchrotron, towards examining the behaviour of actinide materials related to nuclear waste forms under extreme conditions. This presentation will highlight some of our successes in advancing the study of actinide and nuclear materials including • High temperature in situ redox chemistry of uranium using X-ray absorption spectroscopy. • The behaviour of uranium oxides under high pressure ( >6 GPa) using neutron diffraction. • High temperature studies of thorium oxides using synchrotron X-ray and neutron diffraction. • Chemistry of uranium oxides under high purity hydrogen gas reduction and high temperature conditions (1000 ℃) using synchrotron X-ray diffraction.
The unusual structural chemistry of uranium: controlling phase transformations in ternary uranium oxides
(Society of Crystallographers in Australia and New Zealand, 2017-12-03) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Avdeev, M; Kowalski, PM; Brand, HEA; Johannessen, B; Kennedy, BJ
Structural investigations of ternary uranium oxides are pertinent to the management of spent nuclear fuel since such environmentally hazardous and potentially dangerous secondary phases may form during the process and/or storage of the material [1]. They further allow for the exploration of the peculiar, exotic and poorly known properties of materials containing, or which can access 5f electrons. The rhombohedral oxides AUO4 for A = α-Sr or Ca in space group 𝑅𝑅3�𝑚𝑚 exemplifies this. We have found through a combination of in situ synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that α-SrUO4 undergoes a fascinating first-order phase transformation under oxidising conditions [2]. This involves the synergetic loss of lattice oxygen resulting in oxygen vacancy defect formation and reduction of the uranium cations, which seemingly reduces the activation energy barrier for transformation to its orthorhombic form, β-SrUO4. Under similar conditions, CaUO4 does not display any transformative behaviour, however, defects can be engineered through the substitution of Ca2+ for Sr2+ in the solid solution α-SrxCa1-xUO4 when heated to high temperature under oxidising conditions. The introduction of Sr2+ cations in α-SrxCa1-xUO4 was found to decrease the temperature at which oxygen vacancy defects form. This phenomenon was rationalised as a consequence of the introduction of Sr2+ cations leading to lattice expansion, which causes the proximity of defects to increase. This subsequently reduces free energy increasing defect-defect interactions, allowing defects to form at lower temperature. Remarkably, when heated under reducing conditions, the disordered oxygen defect containing rhombohedral α-SrUO4-x structure undergoes a reversible first-order phase transformation that involves the ordering of the oxygen defects resulting in lowering of the crystallographic symmetry to triclinic in space group P 1� denoted δ-SrUO4-x. This remarkable transformation, which implies entropy is being decreased as temperature increases, could be replicated in CaUO4 and also in α-Sr0.4Ca0.6UO4 where the transformation temperature is reduced by increasing the Sr2+ content, consistent with the effects of reducing defect-defect interactions. The S-XRD data shows the structure of δ-CaUO4-x to be incommensurate whereas δ-SrUO4-x is commensurate. This implies a miscibility gap may exist between the isostructural CaUO4 and α-SrUO4 related to shortrange order. This investigation demonstrates the rich and fascinating crystal chemistry present in uranium oxides, which, in some cases, may have profound societal importance if it can either be safely used or if associated properties can be replicated into non-actinide materials.

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