Browsing by Author "Whittle, KR"

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    119Sn MAS NMR and first-principles calculations for the investigation of disorder in stannate pyrochlores
    (Royal Society of Chemistry, 2011-01-14) Mitchell, MR; Reader, SW; Johnston, KE; Pickard, CJ; Whittle, KR; Ashbrook, SE
    The local structure and cation disorder in Y2Ti2−xSnxO7 pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using 119Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how 119Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift; an upfield shift resulting from the incorporation of the more polarizing Ti4+ cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the 119Sn spectra hinders the extraction of the detailed structural information previously obtained using 89Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations. © 2011, Royal Society of Chemistry
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    Crystal structure influence on ion-irradiation tolerance of Ln2TiO5 compounds
    (Engineers Australia, 2014-11-26) Aughterson, RD; Lumpkin, GR; Gault, B; Whittle, KR; de los Reyes, M; Smith, KL; Cairney, JM
    As a controllable and reproducible technique ion beam irradiation is routinely used as a method for simulating recoil damage caused by alpha-decay in actinide containing materials and neutron damage within fission and fusion reactor systems [1]. The transition from a crystalline to amorphous structure may lead to larger scale effects in the material properties such as an increase in volume (swelling) linked to the generation and agglomeration of defects and decreased thermodynamic and structural stability. Compounds in this study consist of the nominal stoichiometry Ln2TiO5 (where Ln represents the lanthanide series plus yttrium). There are 4 crystal structure symmetries in this series; orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The final structure is dependent upon the lanthanide size and processing conditions used (pressure and temperature regime) [2]. Ln2TiO5 compounds have been proposed for potential nuclear based applications [3]. For example, Dy2TiO5 has been incorporated into inert matrix fuel as a neutron absorber [4] and Gd2TiO5 has also been proposed for similar use [5] and it’s radiation tolerance has been tested for this purpose [6]. In this study a systematic series of samples of the nominal stoichiometry Sm(x)Yb(2-x)TiO5 (where x = 2, 1.4, 1, 0.6, and 0) are used to test and compare the ion-irradiation tolerance of the major structures within the Ln2TiO5 system of compounds. An improved radiation tolerance with the higher symmetry cubic structures was found, which is consistent with previous studies of similar compounds.
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    Density and structural effects in the radiation tolerance of TiO2 polymorphs
    (IOP Publishing, 2013-08-05) Qin, MJ; Kuo, EY; Whittle, KR; Middleburgh, SC; Robinson, M; Marks, NA; Lumpkin, GR
    The radiation response of TiO2 has been studied using molecular dynamics. The simulations are motivated by experimental observations that the three low-pressure polymorphs, rutile, brookite and anatase, exhibit vastly different tolerances to amorphization under ion-beam irradiation. To understand the role of structure we perform large numbers of simulations using the small thermal spike method. We quantify to high statistical accuracy the number of defects created as a function of temperature and structure type, and reproduce all the main trends observed experimentally. To evaluate a hypothesis that volumetric strain relative to the amorphous phase is an important driving force for defect recovery, we perform spike simulations in which the crystalline density is varied over a wide range. Remarkably, the large differences between the polymorphs disappear once the density difference is taken into account. This finding demonstrates that density is an important factor which controls radiation tolerance in TiO2. © 2013, IOP Publishing Ltd.
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    Electron backscatter diffraction characterization of plasma immersion ion implantation effects in stainless steel
    (Elsevier, 2013-01-15) Davis, J; Short, KT; Wuhrer, R; Phillips, MR; Lumpkin, GR; Whittle, KR
    In these experiments plasma immersion ion implantation is utilised to simulate some of the radiation effects in a nuclear reactor environment. Scanning electron microscopy using the angular selective backscatter detector has revealed observable changes in crystallographic contrast after irradiation with helium ions. Further studies using electron backscatter diffraction in both plan and cross section view allow us to visualize the extent and depth of damage and observe differences in the behavior of different crystalline phases present in several grades of stainless steel. © 2012, Elsevier B.V.
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    In situ radiation damage studies of Ca3Zr2FeAlSiO12 and Ca3Hf2FeAlSiO12
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2009-11-25) Whittle, KR; Blackford, MG; Smith, KL; Lumpkin, GR; Zaluzec, NJ
    Garnets, A3B2C3O12, are considered to be potential host phases for the immobilization of high-level nuclear waste as they can accommodate a number of elements of interest, including Zr, Ti and Fe. The naturally occurring garnet, kimzeyite, Ca3(ZrTi)2(SiAlFe)O12, can contain ∼30wt% Zr. An understanding of the radiation tolerance of these materials is crucial to their potential use in nuclear waste immobilization. In this study two synthetic analogues of kimzeyite of composition Ca3(ZrTi)2(SiAlFe)O12 and Ca3Hf2FeAlSiO12 were monitored in situ during irradiation with 1.0 MeV Kr ions using the intermediate voltage electron microscope-tandem user facility (IVEM) at Argonne National Laboratory. The structure of these materials was previously determined by neutron diffraction and 57Fe Mössbauer spectroscopy. Ca3(ZrTi)2(SiAlFe)O12 and Ca3Hf2FeAlSiO12 have very similar structural properties with cubic la3d symmetry, the only significant difference being the presence of Zr and Hf, respectively on the 6 coordinated B sites. © 2009 AINSE
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    The influence of crystal structure on ion-irradiation tolerance in the Sm(x)Yb(2-x)TiO5 series
    (Elsevier B.V., 2016-04-01) Aughterson, RD; Lumpkin, GR; de los Reyes, M; Gault, B; Baldo, P; Ryan, EA; Whittle, KR; Smith, KL; Cairney, JM
    This ion-irradiation study covers the four major crystal structure types in the Ln2TiO5 series (Ln = lanthanide), namely orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. This is the first systematic examination of the complete Ln2TiO5 crystal system and the first reported examination of the hexagonal structure. A series of samples, based on the stoichiometry Sm(x)Yb(2-x)TiO5 (where x = 2, 1.4, 1, 0.6, and 0) have been irradiated using 1 MeV Kr2+ ions and characterised in-situ using a transmission electron microscope. Two quantities are used to define ion-irradiation tolerance: critical dose of amorphisation (Dc), which is the irradiating ion dose required for a crystalline to amorphous transition, and the critical temperature (Tc), above which the sample cannot be rendered amorphous by ion irradiation. The structure type plus elements of bonding are correlated to ion-irradiation tolerance. The cubic phases, Yb2TiO5 and Sm0.6Yb1.4TiO5, were found to be the most radiation tolerant, with Tc values of 479 and 697 K respectively. The improved radiation tolerance with a change in symmetry to cubic is consistent with previous studies of similar compounds. © 2016 Elsevier B.V.
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    Investigating radiation damage in ceramics: the role of in situ microscopy
    (Cambridge University Press, 2010-08-01) Smith, KL; Lumpkin, GR; Whittle, KR; Blackford, MG; Zaluzec, NJ
    Radiation damage effects in ceramics are of interest in both current (e.g. nuclear waste forms) and next generation (Gen IV) nuclear technologies (e.g. transmutation targets, inert matrix fuels, waste forms etc.). The mechanisms of radiation damage production and recovery in ceramics often vary dependent on the damage source, e.g., energetic neutrons in reactor systems versus alpha recoil damage in nuclear waste forms. Furthermore, the kinetics of damage recovery are complicated by multiply activated processes and in certain cases, longer-term process may modify the structural state left by irradiation in the short term. Here, we review a selection of data collected by the current authors and associates, which highlight the roles played by in situ microscopy in unraveling the mechanisms of radiation damage in ceramics. In situ irradiation of a variety of ceramics (e.g. pyrochlores, perovskites, polymorphs of TiO2 and analogues etc.) have been carried out using the IVEM-Tandem Facility at Argonne National Laboratory [1-7]. The IVEM-Tandem Facility is comprised of an intermediate voltage TEM (IVEM), a Hitachi H-9000NAR, interfaced to two ion accelerators (at 30° to vertical). Specimens were generally prepared by crushing and suspending small fragments on holey carbon coated Cu grids. Samples were most often, irradiated using 1 MeV Kr ions at temperatures from 50 to 900K. Figure 1 shows (a) a bright field (BF) image and (b) selected area diffraction pattern (SADP) of a grain of a typical specimen (in this case TiO2) before in situ irradiation and (c) the SADP of the same grain after a series of irradiations. Multiple measurements were taken and averaged to ascertain the critical fluence (of particular ions) for amorphisation, Fc, of a given sample at various temperatures. Fc data were then were used to establish the critical temperature, Tc, of the particular sample, where Tc is the temperature at which the recovery process is at least equal to the damage rate (that is the temperature above which a sample can not be amorphised). IVEM-Tandem data on a range of pyrochlores with different compositions have been compiled and used to develop an empirical model that can predict the Tc values for many pyrochlores within +/- 80°C [2, 6]. IVEM-Tandem results for specific solid solution series of pyrochlores are informative and will be discussed more fully in the conference presentation. LaxSr1-3x/2TiO3 perovskites show a non-linear relationship for Tc with x [3]. In combination with other data, in situ TEM suggests that cubic perovskites in the 0
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    Ion beam irradiation of lanthanum compounds in the series La2O3-TiO2
    (Materials Research Society, 2010-04-08) Whittle, KR; Blackford, MG; Aughterson, RD; Smith, KL; Lumpkin, GR; Zaluzec, NK
    Thin crystals of La2O3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions in the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), at the Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1 × 1016 ions cm-2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for TiO2, are discussed with reference to the phase diagrams for La2O3-TiO2 systems and the different local environments within the crystal structures. Results suggest an observable inverse correlation between Tc and melting temperature (Tm) in the two systems.
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    Ion irradiation of ternary pyrochlore oxides
    (American Chemical Society, 2009-07-14) Lumpkin, GR; Smith, KL; Blackford, MG; Whittle, KR; Harvey, EJ; Redfern, SAT; Zaluzec, NJ
    Polycrystalline synthetic samples of Y2Ti2−xSnxO7 with x = 0.4, 0.8, 1.2, and 1.6, together with Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in situ in the intermediate voltage electron microscope (IVEM)-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions at temperatures of 50 to 650 K. Determination of the critical amorphization fluence (Fc) as a function of temperature has revealed a dramatic increase in radiation tolerance with increasing Sn content on the pyrochlore B site. Nonlinear least-squares analysis of the fluence-temperature curves gave critical temperatures (Tc) of 666 ± 4, 335 ± 12, and 251 ± 51 K for the Y2Ti2−xSnxO7 samples with x = 0.4, 0.8, and 1.2, respectively. The sample with x = 1.6 appears to disorder to a defect fluorite structure at a fluence below 1.25 × 1015 ions cm−2 and remains crystalline to 5 × 1015 ions cm−2 at 50 K. Additionally, the critical fluence-temperature response curves were determined for Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7, and we obtained Tc values of 685 ± 53 K and 473 ± 52 K, respectively, for these pyrochlores. Nd2Zr2O7 did not become amorphous after a fluence of 2.5 × 1015 ions cm−2 at 50 K, but there is evidence that it may amorphize at a higher fluence, with an estimated Tc of 135 K. The observed Tc results are discussed with respect to the predicted Tc values based upon a previously published empirical model (Lumpkin, G. R.; Pruneda, M.; Rios, S.; Smith, K. L.; Trachenko, K.; Whittle, K. R.; Zaluzec, N. J. J. Solid State Chem. 2007, 180, 1512). In the Y2Ti2−xSnxO7 pyrochlores, Tc appears to be linear with respect to composition, and is linear with respect to rA/rB and x(48f) for all samples investigated herein. © 2009, American Chemical Society
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    Ion irradiation of ternary pyrochlores
    (Materials Research Society, 2008-12-01) Whittle, KR; Smith, KL; Blackford, MG; Redfern, SAT; Harvey, EJ; Zaluzec, NJ; Lumpkin, GR
    Synthetic pyrochlore samples Y2Ti2-xSnxO7 (x=0.4, 0.8, 1.2, 1.6), Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in-situ using the IVEM-TANDEM microscope facility at the Argonne National Laboratory. The critical temperatures for amorphisation have revealed a dramatic increase in tolerance with increasing Sn content for the Y2Ti2-xSnxO7 series. This change has also found to be linear with increasing Sn content. Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7 were both found to amorphise, while Nd2Zr2O7 was found to be stable to high doses (2.5x10^15 ions cm-2). The observed results are presented with respect to previously published results for irradiation stability predictions and structural disorder. © Materials Research Society 2009
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    Ion irradiation of the TiO2 polymorphs and cassiterite
    (Mineralogical Society of America, 2010-01) Lumpkin, GR; Blackford, MG; Smith, KL; Whittle, KR; Zaluzec, NJ; Ryan, EA; Baldo, P
    Thin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50–300 K. Synthetic rutile and natural cassiterite, with 0.1–0.2 wt% impurities, remain crystalline up to a fluence of 5 x 1015 ions cm–2 without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3–0.5 wt% impurities, become amorphous at fluences of 8.1 x 1014 and 2.3 x 1014 ions cm–2, respectively. We have also studied two natural rutile samples containing ~1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 x 1014 and 8.6 x 1014 ions cm–2 at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 ± 11, 209 ± 8, and 242 ± 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration. © 2010, Mineralogical Society of America
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    Ion-beam irradiation of lanthanum compounds in the systems La2O3–Al2O3 and La2O3–TiO2
    (Elsevier, 2010-10) Whittle, KR; Lumpkin, GR; Blackford, MG; Aughterson, RD; Smith, KL; Zaluzec, NJ
    Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation. © 2010, Elsevier Ltd.
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    Ion-irradiation resistance of the orthorhombic Ln2TiO5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) series
    (Elsevier, 2015-12-01) Aughterson, RD; Lumpkin, GR; Ionescu, M; de los Reyes, M; Gault, B; Whittle, KR; Smith, KL; Cairney, JM
    The response of Ln2TiO5 (where Ln is a lanthanide) compounds exposed to high-energy ions was used to test their suitability for nuclear-based applications, under two different but complementary conditions. Eight samples with nominal stoichiometry Ln2TiO5 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy), of orthorhombic (Pnma) structure were irradiated, at various temperatures, with 1 MeV Kr2+ ions in-situ within a transmission electron microscope. In each case, the fluence was increased until a phase transition from crystalline to amorphous was observed, termed critical dose Dc. At certain elevated temperatures, the crystallinity was maintained irrespective of fluence. The critical temperature for maintaining crystallinity, Tc, varied non-uniformly across the series. The Tc was consistently high for La, Pr, Nd and Sm2TiO5 before sequential improvement from Eu to Dy2TiO5 with Tc's dropping from 974 K to 712 K. In addition, bulk Dy2TiO5 was irradiated with 12 MeV Au+ ions at 300 K, 723 K and 823 K and monitored via grazing-incidence X-ray diffraction (GIXRD). At 300 K, only amorphisation is observed, with no transition to other structures, whilst at higher temperatures, specimens retained their original structure. The improved radiation tolerance of compounds containing smaller lanthanides has previously been attributed to their ability to form radiation-induced phase transitions. No such transitions were observed here. © 2017 Elsevier B.V.
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    Ionisation efficiency improvements for AMS measurement of actinides
    (Elsevier, 2010-04) Child, DP; Hotchkis, MAC; Whittle, KR; Zorko, B
    Measurements of the isotopic ratios of plutonium and uranium are utilised by the safeguards community, environmental radioactivity and remediation studies, oceanography research, and in the U-series dating of geological samples. Accelerator Mass Spectrometry offers the advantages of high selectivity and sensitivity for such ratio measurements, but suffers from a poorer ionisation and transmission efficiency than some other mass spectrometric techniques. In addition, the ionisation efficiency achieved in caesium-sputter ion sources can be variable depending on the chemical form and geometry of the target used, yielding actinide negative ion beam rates with efficiencies ranging from 0.05% to 0.5%. In this paper we investigate the impact of the crystal structure of the target material, chemical composition and the geometry of the target in order to determine conditions which will allow us to maximise beam output and increase the efficiency of AMS measurements in future. We also examine target stability and “burn in time” (the time taken to generate a stable beam current) in conjunction with these measurements in order to maintain a high precision of measurement during the measurement period. © 2009 Published by Elsevier B.V.
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    Lanthanide titanates – atypical co-ordination and stuffed pyrochlores?
    (Australian Institute of Physics, 2009-02-05) Whittle, KR
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    Lanthanum pyrochlores and the effect of yttrium addition in the systems La2-xYxZr2O7 and La2-xYxHf2O7
    (Elsevier, 2009-03) Whittle, KR; Cranswick, LMD; Redfern, SAT; Swainson, IP; Lumpkin, GR
    The crystal structures of the compounds La2−xYxZr2O7 and La2−xYxHf2O7 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,3/8,3/8) 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. © 2009, Elsevier Ltd.
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    M(n+1)AXn phases are they tolerant/resistant to damage
    (Australian Institute of Physics, 2011-02-03) Whittle, KR; Riley, DP; Blackford, MG; Aughterson, RD; Moricca, SA; Lumpkin, GR; Zaluzec, NJ
    Ternary carbide materials have been proposed as having applications within the future nuclear technologies, both fusion (ITER/DEMO) and fission (Gen IV). These new designs require a material to have the ability to tolerate radiation damage to high levels, with a high level of predictability. As part of such a process two systems, specifically Ti3AlC2 and Ti3SiC2 have been studied to determine their radiation tolerance, using in-situ ion beam irradiation with 1 MeV Xe ions, coupled with transmission electron microscopy. Irradiations have shown that both systems show little amorphisation at 300K up to doses of at least 6.25 x 1015 ions cm-2 (~28-30 dpa). However, there is a subtle difference between Ti3AlC2 and Ti3SiC2, with Ti3SiC2 showing more evidence for damage. Further irradiations using 500 KeV Xe to fluences equivalent to 100 dpa have also been undertaken, with crystalline material visible and evidence of recrystallisation. Explanations and possible mechanisms for recovery from damage are presented, along with implications for future potential uses.
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    Mechanisms of radiation damage and properties of nuclear materials
    (Materials Research Society, 2009-11-30) Lumpkin, GR; Smith, KL; Whittle, KR; Thomas, BS; Marks, NA
    A wide range of materials are currently under consideration for use in advanced nuclear fuel cycle applications. The effects of radiation on these materials by exposure to external neutron irradiation and internal alpha and beta decay processes may have significant effects on the physical and chemical properties. This is especially true for materials that are subject to hundreds of displacements per atom during their service life. In this paper, we explore some of the radiation damage mechanisms prevalent in oxide based materials, including mathematical models and other concepts of amorphization (e.g., percolation), the role of defects on picosecond time scales, and longer term effects such as diffusion and recrystallization. As radiation "tolerance" or the ability of a material to maintain crystallinity under intense irradiation is a key issue for many fuel cycle applications, we will briefly review and comment on some of the underlying factors that have been identified as important in driving the short-term damage recovery. These include aspects of the structure (e.g., connectivity, polyhedral distortion), bonding, energetics of defect formation and migration, and melting point and similar criteria. The primary materials of interest here are those under development as special purpose nuclear waste forms, novel materials for separations, inert matrix fuels, and transmutation targets. In this context, we will illustrate the behavior of simple oxides and several more complex oxides such as perovskite, multicomponent fluorite systems, and related derivative structures (e.g., pyrochlore and zirconolite). The damage mechanisms in these materials are briefly compared with those of intermetallic and metallic materials.
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    A model for electron scattering in irradiated pyrochlore-fluorite systems
    (Cambridge University Press, 2009-07-26) Lumpkin, GR; Blackford, MG; Smith, KL; Whittle, KR; Zaluzec, NJ
    Certain minerals, materials for energy, and high technology devices may undergo irradiation, either internally through alpha decay, or externally by neutrons or ions [1]. Many of these materials become amorphous when irradiated beyond a critical dose level. Electron diffraction patterns are routinely used to study the structure of these materials, including the observation of order-disorder phenomena during irradiation [2]. Here we show how the intensities of Bragg beams change from a dynamical to a kinematic-like state during the accumulation of amorphous domains in the material. The transition from the crystalline to the amorphous state can be considered as a two-phase problem, with fc + fa = 1, where fc and fa are the fractions of the crystalline and amorphous phases, both having the same composition. For fc = 1 the perfect crystal scatters into the nodes of the reciprocal lattice according to the many-beam dynamical equations and for fa = 1 the amorphous phase scatters according to the Debye equation. Figure 1 shows the strong Bragg-Bragg interactions that occur in the perfect crystal according to relationships like (hkl)1 + (hkl)2 = (hkl)3. However, once there are sufficient amorphous domains in the material, the diffuse scattering from these domains will begin to attenuate the double diffraction relationship between Bragg beams via relationships like (hkl)1 + (am)1 = (am)2 and (am)1 + (hkl)1 = (am)2. Multiple diffuse scattering, e.g., (am)1 + (am)2 = (am)3, will eventually come into play as shown in Figure 1. Thus, the relative intensities of the Bragg beams change dramatically with increasing radiation dose as fa increases. The above model also suggests that the intensity scattered from the amorphous phase will be dependent upon the orientation of the specimen and that Bragg-diffuse interactions may be observed. We tested this model by irradiating TEM samples of Gd2Ti2O7 in the IVEM-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions. The top row of Figure 2 reveals a systematic change in the Bragg beam intensities of the crystalline fraction from a dynamical pattern toward a kinematic-like pattern for the [110] zone axis with increasing dose. The weaker superlattice beams are lost in the amorphous background or in the enhanced small angle scattering. As shown previously [3], kinematically strong beams always the last to disappear during irradiation. Additional dynamical scattering effects are shown in the bottom row of Figure 2, in which the specimen is tilted to a 〈111〉 systematic row. These SAED patterns exhibit stronger scattering at lower dose levels from the amorphous fraction due to the removal of numerous Bragg beams from the Ewald sphere upon tilting, together with multiple Bragg-diffuse scattering along the systematic row. Certain superlattice beams (e.g., 333) can be observed to high dose levels, indicating that the crystalline domains retain the ordered pyrochlore structure. The model provides a general description of changes in Bragg beam intensities during the crystalline-amorphous transformation, but it is especially relevant to pyrochlore-fluorite systems in which the superlattice beams are very weak in the kinematic-like state. This has major implications for the interpretation of order-disorder phenomena for different damage mechanisms. References [1] Ewing, R.C., Weber, W.J., Lian, J., J. Appl. Phys. 95, 5949-5972 (2004). [2] Lian, J., Chen, J., Wang, L.M., Ewing, R.C., Farmer, J.M., Boatner, L.A., Helean, K.B., Phys. Rev. B 68, 134107 (2003). [3] Smith, K.L., Zaluzec, N.J., Lumpkin, G.R., In situ studies of ion irradiated zirconolite, pyrochlore, and perovskite. J. Nucl. Mater. 250, 36-52 (1997). © 2009 Microscopy Society of America
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    Neutron and resonant x-ray diffraction studies of zirconolite 2M
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2009-11-25) Whittle, KR; Lumpkin, GR; Smith, KL; Hyatt, NC
    Zirconolite (nominally CaZrTi2O7) is a constituent phase of potential waste forms for the safe immobilisation of actinide wastes. Structural studies of such materials provide important information about cation ordering, lattice parameters, and strain effects, and provide input into the modeling of alpha decay damage and the development of future wasteform designs. A suite of zirconolites based on the replacement of Ti with Nb and Fe has been studies using high resolution neutron diffraction and resonant x-ray diffraction to determine the degree of disorder across the available cation sites. Resonant x-ray diffraction is a unique method which allows the location of certain cations to be to be determined accurately by taking advantage of the change in scattering power close to an absorption edge (e.g.,, Nb-K and Zr-K). Using standard x-ray diffraction alone this is not possible and there is little scattering difference between Nb and Zr. Previous results on these materials have shown that the majority of the CaZrTi2-2xNbxFexO7 solid-solution series adopt the 2M polytype structure. Raman spectroscopy and measured lattice parameters have shown that the exchange of Ti with Nb+Fe has a non-linear effect on the unit cell dimensions while retaining the 2M polytype. Mössbauer spectroscopy has shown that the Fe preferentially fills the Ti split (Ti2) site. The new results provide a more complete picture of the cation order-disorder problem and are generally consistent with the behaviour of lattice parameters across the series. © 2009 AINSE