Bulk-sensitive photoemission spectroscopy of A_2FeMoO_6 double perovskites (A=Sr, Ba)

Electronic structures of Sr_2FeMoO_6 (SFMO) and Ba_2FeMoO_6 (BFMO) double perovskites have been investigated using the Fe 2p->3d resonant photoemission spectroscopy (PES) and the Cooper minimum in the Mo 4d photoionization cross section. The states close to the Fermi level are found to have strongly mixed Mo-Fe t_{2g} character, suggesting that the Fe valence is far from pure 3+. The Fe 2p_{3/2} XAS spectra indicate the mixed-valent Fe^{3+}-Fe^{2+} configurations, and the larger Fe^{2+} component for BFMO than for SFMO, suggesting a kind of double exchange interaction. The valence-band PES spectra reveal good agreement with the LSDA+U calculation.Comment: 4 pages, 3 figure

The importance of the magnetic dipole term in magneto-circular x-ray absorption dichroism for 3d transition metal compounds

The application of the magneto-circular x-ray absorption dichroism (MCXD) spin sum rule for 3d transition metal compounds faces two problems: the unknown value of the magnetic dipole operator T-z and the division between the L(2) and L(3) edges. A systematic study of the order of magnitude of the T-z-operator for 3d(n) ions is presented. The variation of the T-z-values with temperature is described and analysed, for all cases from d(1) to d(9) cations in two different situations. Firstly the perfect octahedral case is considered. It is shown that T-z is non-zero for low temperature; but, as it originates only from d-electron spin-orbit splitting, it is washed out at room temperature. Secondly, a model of the surface situation is considered. In this case T-z originates mainly from the crystal-field splitting. It then exhibits quite large values at any temperature and can by no means be neglected when applying the sum rule. The error introduced in the sum rule due to the mixing of L(2) and L(3) edges has been estimated

Vers la modélisation de cascades de déplacements dans la zirconolite : détermination de potentiels empiriques

La zirconolite (CaZrTi2O7) est envisagée comme matrice cristalline de confinement des actinides (Pu, Cm, Am). Ceux-ci émettent, lors des désintégrations β, des noyaux de recul qui initient des cascades de collisions atomiques au sein du matériau. Ces cascades modifient les caractéristiques structurales de la matrice (amorphisation, gonflement...). Il est possible de modéliser à l'échelle atomique les effets des noyaux de recul sur un matériau par dynamique moléculaire (DM). La DM permet de simuler numériquement le mouvement des atomes en intégrant les équations de Newton. Afin de calculer les forces qui agissent sur chaque particule, on a besoin d'un potentiel d'interaction qui modélise le matériau. Entre les ions de charge Zie et Zje, nous employons le potentiel empirique de paire dépendant de la distance interatomique [MATH]. Les deux premiers termes forment le potentiel d'interaction à courte portée de Buckingham. Les paramètres du potentiel (Aij, ρij, Cij), caractéristiques des paires de types d'atomes, sont ajustés de manière empirique par des méthodes itératives, c'est-à-dire par minimisation des différences entre les propriétés physiques expérimentales et calculées à partir des potentiels. Cette phase préliminaire est déterminante pour les simulations ultérieures de cascades. Nous présentons ici un jeu de paramètres qui modélise au mieux les propriétés physiques de la zirconolite

Radiation damage in materials: coupling X-ray diffraction with multi-scale numerical simulations

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Differences in coupling between nuclear and electronic energy losses in UO2 with irradiation temperature: an in situ TEM study

International audienceTo investigate the coupling between nuclear and electronic energy losses in UO2, we irradiated thin foils with 0.39 MeV Xe and/or 6 MeV Si ions at 93 K using single or simultaneous dual beam ion irradiations. The evolution of perfect dislocation loops was characterized by in situ transmission electron microscopy (TEM). Additional ex situ TEM characterizations at room temperature revealed for the first time in UO2 the presence of faulted Frank loops too small to be measured during in situ experiments and conventional bright field kinematical imaging conditions. For the single Xe irradiation, which favor dominant ballistic energy losses, we observed a continuous nucleation of small perfect dislocation loops, which increase in size for our last fluences by growing through mainly coalescence effect. Both the single Si and dual Xe & Si irradiations showed a coupling between nuclear and electronic energy losses, resulting in a significant loop density increase and a tangled line network formation, respectively. These phenomena occur at lower dpa levels, compared to the single Xe irradiation, likely resulting from the thermal spike effect of Si ions. The present results were compared to our previous work at 293 K to investigate the role of irradiation temperature on the energy losses coupling. For the Xe irradiation, the density increases and the loops are smaller at 93 K compared to 293 K, resulting from the uranium interstitials mobility being prevented or allowed. For the Si irradiation, the dislocation evolution kinetics are similar at both temperatures. The electronic excitations effect seems greater than the irradiation temperature effect in this temperature range. For the Xe & Si irradiation, the loop kinetics change resulting in a tangled line network formation is faster and thus the loop transformation into lines occurs at lower dpa levels at 93 K compared to 293 K. It appears that the irradiation temperature affecting the mobility of some small point defects reduces the electronic excitation effect in this case

Damage processes in MgO irradiated with medium-energy heavy ions

International audienceThe micro-structural modifications produced in MgO single crystals exposed to medium-energy heavy ions (1.2-MeV Au) were investigated using Rutherford backscattering spectrometry in channeling geometry coupled to Monte-Carlo analyses, secondary ion mass spectrometry, X-ray diffraction and transmission electron microscopy. The damage accumulation and the elastic strain variation were interpreted in the framework of the multi-step damage accumulation (MSDA) model. Both build-ups follow a multi-step process similar to that recently observed for ion-irradiated yttria-stabilized zirconia (YSZ) single crystals. However, in MgO, an unexpectedly high disorder level occurs far beyond the theoretical damage distribution. These results strongly suggest that the migration of defects created in the near-surface layer is most likely at the origin of the broadening of the damage depth distribution in MgO

Study of self-limiting oxidation of silicon nanoclusters by atomistic simulations

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Key role of the short-range order on the response of the titanate pyrochlore Y 2 T i 2 O 7 to irradiation

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