title: A detailed Raman and X-ray study of UO2+xoxides and related structure transitions.
abstract: This work presents a detailed study of hyperstoichiometric UO2+x(0 < x < 0.25) oxides and an assessment of the structural evolution taking place as oxidation proceeds. For this purpose, different UO2+xpowder samples with controlled degree of non-stoichiometry have been identified by thermogravimetric analysis and characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD analysis reflects that the commonly assumed Vegard's law is not applicable over the whole hyperstoichiometry range, since a slight increase of the lattice constant is observed for 0.13 < x < 0.20. A quantitative Raman analysis of the UO2+xspectra as a function of the oxidation degree is also shown. A new method to characterize any UO2+xsample (for x < 0.20), based on the shift of the 630 cm-1band observed in the Raman spectrum, is proposed here for the first time. Moreover, three structure transitions have been detected at x = 0.05, 0.11 and 0.20, giving rise to four distinct regions associated with consecutive structural rearrangements over the hyperstoichiometry range: x < 0.05, 0.05 < x < 0.11, 0.11 < x < 0.20 and 0.20 < x < 0.25.
authors.affiliation: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT Avenida Complutense, Departamento de Energía, Unidad de Residuos de Alta Actividad, 40-28040, Madrid, Spain. jonemiren.elorrieta@ciemat.es.
authors.forename: J M
authors.initials: JM
authors.lastname: Elorrieta
authors.affiliation: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT Avenida Complutense, Departamento de Energía, Unidad de Residuos de Alta Actividad, 40-28040, Madrid, Spain. jonemiren.elorrieta@ciemat.es.
authors.forename: L J
authors.initials: LJ
authors.lastname: Bonales
authors.affiliation: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT Avenida Complutense, Departamento de Energía, Unidad de Residuos de Alta Actividad, 40-28040, Madrid, Spain. jonemiren.elorrieta@ciemat.es.
authors.forename: N
authors.initials: N
authors.lastname: Rodríguez-Villagra
authors.affiliation: MALTA-Consolider Team, Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain.
authors.forename: V G
authors.initials: VG
authors.lastname: Baonza
authors.affiliation: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, CIEMAT Avenida Complutense, Departamento de Energía, Unidad de Residuos de Alta Actividad, 40-28040, Madrid, Spain. jonemiren.elorrieta@ciemat.es.
authors.forename: J
authors.identifier: http://orcid.org/0000-0002-2048-5282
authors.initials: J
authors.lastname: Cobos