Recently, unusual Raman signals were observed in different works conducted on thin zirconia layers grown on zirconium alloys simulating in-reactor materials after high fluence ion irradiation or for samples cut from fuel rods irradiated in nuclear plants. As such, these spectra clearly do not correspond to any known spectrum of a pure standard zirconia phase. Therefore, the analysis conducted in this paper aims to provide a better understanding of these peculiar Raman features. For that purpose, specific ion-irradiated samples were analysed. In situ Raman spectroscopy was first used to follow the irradiation process. Then, samples were characterized using different excitation wavelengths. Finally, the effects of oxygen isotopic substitution were examined in details. Results are discussed in terms of disorder and size-related effects.

The damage induced in uranium dioxide (UO 2) during ion irradiation at low energy was studied by micro-Raman spectroscopy. Polycrystalline UO 2 samples were irradiated by 0.9-MeV I, 2-MeV Au at 25°C and by 4-MeV Kr ions at-160°C in a wide range of fluence. In situ Raman measurements reveal similar spectra evolution no matter the ion beam used. The T 2g band centred at 445 cm-1 related to the fluorine structure reveals a broadening with the irradiation damage increase. In addition, several bands ranging from 500 to 700 cm-1 , which are attributed to sub-or sur-stoichiometric structural defects, are observed at the first time of irradiation. Their intensities rise up with the irradiation fluence increase to a similar asymptotic relative values for all the irradiation conditions. The obtained Raman kinetics are compared with data from the literature on the microstructure evolution observed by Transmission Electronic Microscopy (TEM) and on the fraction of displaced atoms determined by Rutherford Backscattering Spectroscopy in channelling mode (RBS-C).

The damage induced in cerium dioxide by swift heavy ion irradiation was studied by micro-Raman spectroscopy. For this purpose, polycrystalline sintered pellets were irradiated by 100-MeV Kr, 200-MeV Xe, 10-MeV, and 36-MeV W ions in a wide range of fluence and stopping power (up to ∼28 MeV μm−1). No amorphization of ceria was found whatsoever, as shown by the presence of the peak of Raman-active T2g mode (centered at 467 cm−1) of the cubic fluorite structure for all irradiation conditions. However, a clear decrease of the T2g mode peak intensity was observed as a function of ion fluence to an asymptotic relative value of about 45%. Similar decays were also observed for satellite peaks and second-order peaks. Track radii deduced from the decay kinetics for the 36-MeV W ion data are in good agreement with previous determinations by X-ray diffraction and reproduced by the inelastic thermal spike model for low ion velocities. However, interaction between the nuclear and electronic stopping powers is needed to describe the decay kinetics of 10-MeV W ion data by the thermal spike process. Moreover, the asymmetrical broadening of the main T2g peak after irradiation was analyzed with different theoretical models.

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Boron carbide, a lightweight, high temperature material, has various applications as a structural material and as a neutron absorber. The large solubility range of carbon in boron, between ≈ 9 % and 20 %, has been theoretically explained by some of us by the thermodynamical stability of three icosahedral phases at low temperature, with respective carbon atomic concentrations: 8.7 % (B 10.5 C, named OPO 1 ), 13.0 % (B 6.7 C, named OPO 2 ), whose theoretical Raman spectra are still unknown, and 20 % (B 4 C), from which the nature of some of the Raman peaks are still debated. We report theoretical and experimental results of the first order, non-resonant, Raman spectrum of boron carbide. Density functional perturbation theory enables us to obtain the Raman spectra of the OPO 1 and OPO 2 phases, which are perfectly ordered structures with however a complex crystalline motif of 414 atoms, due to charge compensation effects. Moreover, for the carbon-rich B 4 C, with a simpler 15-atom unit cell, we study the influence of the low energy point defects and of their concentrations on the Raman spectrum, in connection with experiments, thus providing insights into the sensitivity of experimental spectra to sample preparation, experimental conditions and setup. In particular, this enables us to propose a new structure at 19.2 % atomic carbon concentration, B 4.2 C, that, within the local density approximation of density functional theory (DFT-LDA), lies very close to the convex hull of boron carbide, on the carbon-rich side. This new phase, derived from what we name the "3+1" defect complex, helps in reconciling the experimentally observed Raman spectrum with the theory around 1000 cm −1 . Finally, we predict the intensity variations induced by the experimental geometry and quantitavely assess the localisation of bulk and defect vibrational modes and their arXiv:2105.02321v1 [cond-mat.mtrl-sci] 5 May 2021 character, with an analysis of "chain" and "icosahedral" modes.

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