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Based on benchmark synchrotron powder X-ray diffraction data, the subtleties in the electron density of diamond are explored. A contraction of the core density inherently linked to covalent bond formation is experimentally observed.
abstracts
The resent progress in powder diffraction provides data of quality beyond multipolar modeling of the valence density. As was recently shown in a benchmark study of diamond by Bindzus et al.[1] The next step is to investigate more complicated chemical bonding motives, to determine the effect of bonding on the core density. Cubic boron nitride lends itself as a perfect candidate because of its many similarities with diamond: bonding pattern in the extended network structure, hardness, and the quality of the crystallites.[2] However, some degree ionic interaction is a part of the bonding in boron nitride, which is not present in diamond. By investigating the core density in boron nitride we may obtain a deeper understanding of the effect of bonding on the total density. We report here a thorough investigation of the charge density of cubic boron nitride with a detailed modelling of the inner atom charge density. By combining high resolution powder X-ray diffraction data and an extended multipolar model an experimental modeling of the core density is possible.[3] The thermal motion is a problem since it is strongly correlated to the changes of the core density, but by combining the average displacement from a Wilson plot and a constrained refinement, a reasonable result has been obtained. The displacement parameters reported here are significantly lower than those previously reported, stressing the importance of an adequate description of the core density. The charge transfer from boron to nitrogen clearly affects the inner electron density, which is evident from theoretical as well as experimental result. The redistribution of electron density will, if not accounted for, result in increased thermal parameters. It is estimated that 1.7-2 electrons is transferred from boron to nitrogen.
abstracts
Advancements within the field of synchrotron powder X-ray diffraction (SPXRD) have rendered it a viable approach for probing subtle electronic features. It is especially powerful for highly crystalline inorganic extended materials, resolving severe extinction issues conventionally encountered with single-crystal diffraction. Furthermore, SPXRD data exhibit markedly reduced absorption and may be collected in a single exposure. The latter prevents systematic errors from merging a multitude of detector frames, each possessing a slightly different scale factor. These experimental advantages are counterbalanced by a more complicated data analysis, where the key problems are the inherent challenges of overlapping reflections and background subtraction. To evaluate the performance of SPXRD and to test the methodologies for estimating charge densities (CDs), we use benchmark data on diamond collected to low d-spacing. [1] The critical step is the recovery of observed structure factors from the powder pattern. This is the focal point of the present study, scrutinizing several traditional and novel approaches that deviate strongly in terms of model complexity and structure-dependency. All recovered sets of structure factors are evaluated with respect to their capability to determine the true atomic displacement parameter and to estimate the CD by both multipolar modelling and maximum entropy reconstruction. The data are of such exceptional quality that they even reveal how the innermost electron density responds to the formation of covalent bonding (figure below). Supporting their huge success over the last decades, only the Rietveld-based approaches are capable of quantifying this fine feature. The study of core polarization has emerged as a new frontier in chemical bonding studies, and the relevant experimental information may be reliably accessed by SPRXD for highly crystalline materials.
