Modelling the experimental electron density : only the synergy of various approaches can tackle the new challenge

International audienceElectrondensity is a fundamental quantity that enables understanding of the chemical bonding in a molecule or in a solid and the chemical/physical property of a material. Because electrons have a charge and a spin, two kinds of electron densities are available. Moreover, because electron distribution can be described in momentum or in position space, charge and spin density have two definitions and thez can be observed through Bragg (for the position space)or Compton (for the momentum space) diffraction experiments, using X-rays (charge density) or polarized neutrons (spin density). In recent years, we have witnessed many advances in this fiels, stimulated by the increased power of experimental techniques. However, an accurate modelling is still necessary to determine the desired functions from the acquired data. The improved accuracy of measurements and the possibility to combine information from differentexpirenmental techniques require even more flexibility of the models.In this short review, we analyse some of the most important topics that have emerged in the recent literature, especially the most thought-provoking at the recent IUCr general meeting in Montreal

Recent advances in understanding the similarities and differences of colombian euclases

Colombian euclase is rare and associated with emerald in medium-temperature hydrothermal veins hosted by Lower Cretaceous black shales (BS). The original sources of euclase production were the mining districts of Gachala and Chivor in the eastern emerald belt, but in 2016 euclases were also found at the La Marina mine in the western emerald belt. The present study is centered on a chemical and mineralogical examination of zoned Colombian euclase sold on the gem market as "trapiche'. Its texture is characterized by growth bands and sectors distinguished by the presence of numerous inclusions (mainly pyrite, carbonates, and organic matter) which represent around 0.2% of the total volume of the crystals. Xray computed tomography showed that the largest inclusions are randomly located, whereas the small inclusions are concentrated in the center of the crystals, along the crystallographic b axis, between neighboring growth sectors and between growth bands in each sector. The texture cannot be defined as "trapiche', like that of Colombian emeralds, because there is no matrix material from the surrounding BS trapped between the growth sectors and accumulated as dendrites. Three-phase fluid inclusions (FI) containing halite, liquid, and vapor phases are also observed in the euclase, and their volume is identical to that of the inclusions in emerald. Chromium and vanadium are the main chromophores, and the highest concentrations (1240 and 400 ppm, respectively) were found in deep blue-colored zones. Surprisingly, the euclase crystals have high Ge contents, between 230 and 530 ppm. The Rare Earth Element (REE) patterns of euclase are inherited from the enclosed BS or albitized and carbonatized BS. Euclase has the same REE pattern as emerald from the Gachala mines with an Eu anomaly (Eu/Eu* similar to 0.40) and a depletion in Heavy Rare Earth Elements (HREE). The present study allows for the reconstruction of the formation conditions of "trapiche' euclase and discussion about its probable geographic origin, i.e., the eastern emerald belt

Les rayon X et les neutrons se combinent pour révéler la densité résolue en spin

International audienceLa distribution des électrons dans un cristal peut être reconstruite avec une grande précision au moyen de la diffraction des rayons X à haute résolution, alors que la diffraction des neutrons polarisés en spin permet de retrouver la densité d’aimantation. Bien que les grandeurs ainsi mesurées soient toutes deux une traduction du comportement des électrons, ces deux types d’expérience de diffusion sont de nos jours interprétées par des modèles différents.Nous retraçons ici les étapes ayant conduit à la première détermination expérimentale de la densité d’électrons résolue en spin par un traitement combiné des données de diffraction de rayons X et de neutrons polarisés

Electron-density studies of magnetic di-nuclear complexes

International audienceIn the aim to rationalise the conception of single molecular magnets the first step is to explore the interactions in molecular magnetic complexes and to understand their role. To this end we modeled the experimental electron density distributions in di-nuclear complexes. For example, we studied a cobalt(II) compound ([Co2(sym-hmp)2](BPh4)2) which was theoretically studied by Tone et al. in 2007[1]. When decreasing the temperature, the magnetic susceptibility of this complex deviates from the Curie law (Fig. 1) because of the anti-ferromagneticexchange interaction, but the susceptibility increases sharply at low temperature (< 20K). The theoretical analysis of Tone et al. showed that this behavior is explained by a tilt of local distortion axes around cobalt atoms and not by a paramagnetic impurity. A polarized neutron diffraction experiment was carried out in order to model the spin density and verify this hypothesis (Borta et al. (2011), accepted in Phys. Rev. B.)To support this electronical approach and to better describe the metal-ligand interactions, we determined the charge density of this complex using high resolution X-ray diffraction at 100 K. We will present our multipolar model[2] and its description using various tools(Fig. 2). The different interactions will be described and comparison will be made with spin density results from polarized neutron diffraction experiments. We will finally introduce our project of a new program for joint refinements of a unique electronic model based on X-rays and polarized neutrons diffraction data