Browsing by Author "Chang, S"

Now showing 1 - 4 of 4
  • No Thumbnail Available
    Item
    Insight into disorder, stress and strain of radiation damaged pyrochlores
    (European Association of Geochemistry and the Geochemical Society (Goldschmidt), 2021-07-07) Finkeldei, SC; Chang, S; Brandt, F; Bosbach, D; Ionescu, M; Avdeev, M; Simeone, D; Thorogood, GJ
    A2B2O7 pyrochlores are considered as potential nuclear waste forms for e.g. plutonium immobilization. Once radionuclides are immobilized within these waste forms, self-irradiation can cause order/disorder transitions of these nuclear waste form candidates. The aim of this study was to detect the early onset of phase transformation in relation to stress and strain of different pyrochlores: titanate based pyrochlores which are prone to amorphization, and zirconate based pyrochlores that are known to undergo a transformation to defect fluorite. Helium ion irradiation experiments of the two pyrochlore types were conducted at 2-6 MeV with a fluence of 1 x 1016 ions/cm2 resulting in approximately 1 dpa to study the onset of disorder in thin samples, to mimic bulk damage. A combined approach of synchrotron X-ray diffraction (XRD), neutron diffraction, and transmission electron microscopy (TEM) as well as electron backscattered diffraction (EBSD) techniques was utilized to understand the structural and microstructural response towards radiation. Irradiation of the Er2Ti2O7 ceramics resulted in a partially fractured sample most likely caused by strain relief. Neutron diffraction data revealed the formation of amorphized regions whereas some crystallinity remained without signs of stress or strain after irradiation. For Nd2Zr2O7, ion irradiation caused the formation of two pyrochlore phases instead of the expected pyrochlore to defect fluorite structural transition. One of the phases appeared to be very similar to the undamaged material while neutron scattering revealed residual stress in the second pyrochlore phase with a significantly reduced lattice parameter compared to the unirradiated Nd2Zr2O7 sample. While we did not observe indications of strain in either of the phases evidence of an increase in disorder of both the cations and anions was observed.
  • Thumbnail Image
    Item
    Insight Into disorder, stress and strain of radiation damaged pyrochlores: a possible mechanism for the appearance of defect fluorite
    (Frontiers Media S.A., 2021-11-08) Finkeldei, SC; Chang, S; Ionescu, M; Oldfield, DT; Davis, J; Lumpkin, GR; Simeone, D; Avdeev, M; Brandt, F; Bosbach, D; Klinkenberg, M; Thorogood, GJ
    We have examined the irradiation response of a titanate and zirconate pyrochlore—both of which are well studied in the literature individually—in an attempt to define the appearance of defect fluorite in zirconate pyrochlores. To our knowledge this study is unique in that it attempts to discover the mechanism of formation by a comparison of the different systems exposed to the same conditions and then examined via a range of techniques that cover a wide length scale. The conditions of approximately 1 displacement per atom via He2+ ions were used to simulate long term waste storage conditions as outlined by previous results from Ewing in a large enough sample volume to allow for neutron diffraction, as not attempted previously. The titanate sample, used as a baseline comparison since it readily becomes amorphous under these conditions behaved as expected. In contrast, the zirconate sample accumulates tensile stress in the absence of detectable strain. We propose this is analogous to the lanthanide zirconate pyrochlores examined by Simeone et al. where they reported the appearance of defect fluorite diffraction patterns due to a reduction in grain size. Radiation damage and stress results in the grains breaking into even smaller crystallites, thus creating even smaller coherent diffraction domains. An (ErNd)2(ZrTi)2O7 pyrochlore was synthesized to examine which mechanism might dominate, amorphization or stress/strain build up. Although strain was detected in the pristine sample via Synchrotron X-ray diffraction it was not of sufficient quality to perform a full analysis on. Copyright © 2021 Finkeldei, Chang, Ionescu, Oldfield, Davis, Lumpkin, Simeone, Avdeev, Brandt, Bosbach, Klinkenberg and Thorogood.
  • No Thumbnail Available
    Item
    Insight into the order-disorder transition of irradiated pyrochlore solid solutions as potential nuclear waste forms
    (European Association of Geochemistry and the Geochemical Society (Goldschmidt), 2021-07-07) Chang, S; Thorogood, GJ; Avdeev, M; Brandt, F; Bosbach, D; Ionescu, M; Simeone, D; Finkeldei, SC
    Pyrochlores (A2B2O7), a common mineral that can uptake large quantities of actinides, are studied as potential nuclear waste forms to immobilize radionuclides. Many have studied pyrochlore solid solutions where different cations on the A or B site are mixed to imitate waste loading of e.g. Pu within the pyrochlore structure. As a response to irradiation or self-irradiation, zirconate (B=Zr) pyrochlores are known to undergo an order/disorder phase transition into a disordered defect fluorite structure while titanate (B=Ti) pyrochlores become amorphous, however the dominating factor towards phase transition of each is unclear in a solid solution. To gain insight into order-disorder processes, the stress and strain of Nd2Zr2O7 and Er2Ti2O7 endmembers and a (ErNd)2(TiZr)2O7 solid solution were examined. Prior and upon irradiation Nd2Zr2O7 and Er2Ti2O7 both showed no strain. With regards to (ErNd)2(TiZr)2O7, it appears that the synthesis method employed was not sufficient to achieve a 50:50 ratio of A and B cations according to the diffraction results. However, the solid solution contained strain while Nd2Zr2O7 and Er2Ti2O7 was strain free. Further synthesis approaches of (ErNd)2(TiZr)2O7 solid solution members are under way to understand the irradiation response of such mixed A and B site pyrochlores.
  • No Thumbnail Available
    Item
    Zener double exchange from local valence fluctuations in magnetite
    (American Physical Society, 2007-12-14) McQueeney, RJ; Yethiraj, M; Chang, S; Montfrooij, W; Perring, TG; Honig, JM; Metcalf, P
    Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below Tv=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (similar to 80 meV) are shifted upwards in energy above Tv due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Δ5 symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above Tv. © 2007, American Physical Society