Structural relaxation around substitutional Cr3+ in MgAl2O4

The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement to the experiment. The Cr-O distance is close to that in MgCr2O4, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4 solid solution. Despite some angular site distortion, the local D3d symmetry of the B-site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accomodated by strain-induced bond buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three four-fold axes of the cubic structure.Comment: 7 pages, 4 figure

Le désir du dialogue : examen du dispositif énonciatif des Lettres portugaises de Guilleragues

International audienceCette brève contribution se propose de revenir sur un roman épistolaire singulier de l'année 1669 Les Lettres portugaises de Guilleragues pour en interroger le fonctionnement intrinsèque avec les outils de la linguistique de l'énonciation. L'objectif est d'apporter des éléments permettant de caractériser la littérature épistolaire, et plus particulièrement le roman par lettres, à partir de sa scène d'énonciation (ses paramètres énonciatifs conditionnés par le genre) et de sa scénographie (ses indices de mise en mots, de mise en discours). Ce texte dresse sur le devant de la scène un sujet passionnel, qui, dans un bref lamento, déployé en cinq moments d'une rare densité, cherche sa vérité dans son propre discours amoureux à défaut de le trouver dans celui qui l'a fait naître. Nous nous proposons d'examiner certains moments de ce discours, qui sont des instants de vacillement ou d'affirmation, partiel ou total, de la subjectivité, lieux privilégiés d'un dire sans cesse tourné vers l'autre, devenu vital. La lettre, ou les jeux de rôle entre subjectivité et désubjectivation, (re)construction et déconstruction de l'identité à soi et à l'autre, dernier vecteur du discours amoureux, tel est l'enjeu de cet article, qui met au service de l'herméneutique textuelle les typologies des linguistiques de l'énonciation pour tenter de mettre en lumière quelques caractéristiques constitutives du discours épistolair

Fe-speciation in kaolins : a diffuse reflectance study

Abstract--Diffuse r flectance spectra of kaolins have been recorded in samples from different environ-ments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, f rrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding tothe 2QA0 ~ 2(4T~(4G)) transition. The systematic com-parison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments ofcrystal field transitions togoethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination f the Fe-phases associated to kaolins assists in the interpretation f the formation conditions of these minerals. Key Words--Diffuse r flectance spectroscopy, Fe-speciation, Kaolin, Second erivative spectra

Intermediate range order in (Fe,Al) silicate network glasses: a neutron diffraction and EPSR modeling investigation

The local structural environment and the spatial distribution of iron and aluminum ions in sodosilicate glasses with composition NaFexAl1-xSi2O6 (x = 1, 0.8, 0.5 and 0) is studied by high-resolution neutron diffraction combined with structural modeling using the Empirical Potential Structure Refinement (EPSR) code. This work gives evidence of differences in the structural behavior of Al3+ and Fe3+, which are both often considered to act as network formers in charge-balanced compositions. The short-range environment and the structural role of the two cations are not composition dependent, and hence the structure of intermediate glasses can then be seen as a mixture of the structures of the two end-members. All Al3+ is 4-coordinated for a distance d[4]Al3+-O=1.76$\pm$0.01{\AA}. The high-resolution neutron data allows deciphering between two populations of Fe. The majority of Fe3+ is 4-coordinated (d[4]Fe3+-O=1.87$\pm$0.01{\AA}) while the remaining Fe3+ and all Fe2+ (~12% of total Fe) are 5-coordinated (d[5]Fe-O=2.01$\pm$0.01{\AA}). Both AlO4 and FeO4 are randomly distributed and connected with the silicate network in which they share corners with SiO4 tetrahedra, in agreement with a network-forming role of those species. On the contrary FeO5 tends to form clusters and to share edges with each other. 5-coordinated Fe is interpreted as network modifier and it turns out that, even if this coordination number is rare in crystals, it is more common in glasses in which they can have a key role on physical properties