Browsing by Author "Garcia, P"

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    Phase separation in fluorite-related U 1– y Ce y O 2– x : new insights via variable temperature neutron diffraction
    (CEA, 2024-09-01) Simeone, D; Deschanels, X; Garcia, P; Avdeev, M; Ablott, TA; Thorogood, GJ
    The phase separation in the U1-yCey O2-x system for values of y between approximately 0.34 and 0.5 observed at low temperatures (below circa 600 K) purportedly involves only fluorite structures [1,3]. Therefore, to confirm this assumption it is logical to employ high resolution diffraction techniques that can track the progression of the oxygen-sub lattice and resolve peaks that may be overlapping. In this study, the phase separation in the U0.54Ce0.46O2-x system has been revisited using variable temperature high resolution neutron diffraction on samples that are sealed under Ar to prevent a change in oxidation state when heated. As neutron scattering lengths for Unat and O do not differ to a large degree, U coherent cross section 8.903 barn vs O coherent cross section 4.232 barn, information about the oxygen sub lattice can be obtained from neutron diffraction patterns. This is not the case for X-ray diffraction which has been used for the bulk of the studies on this system. Below a critical temperature, the existence of two fluorite related structures in the miscibility gap is confirmed: a stoichiometric U0.54Ce0.46O2 phase and an oxygen-deficient U0.54Ce0.46O2-x phase. Although the former is indeed a fluorite, we show that the other end-member phase has a C-type bixbyite structure, Figure 1. © The Authors
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    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.