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Abstract: Protic Ionic Liquids (PIL) prepared from super-strong base 1,7-diazabicyclo[5.4.0]undec-7-ene (DBU) and super-strong acids, trifluoromethane sulfonic acid (TfOH) and (trifluoromethanesulfonyl)-(nonafluorobutylsulfonyl)imide, (IM14H), have been prepared ([DBUH][TfO] and [DBUH][IM14], respectively; the latter for the first time) and their chemical physical properties and structural features been explored using a synergy of experimental and computational tools. The short range order in neat DBU, as well as the long range structural correlations induced by charge correlation and hydrogen bonding interactions in the ionic liquids, have been explored at ambient conditions, where these compounds are proposed for a variety of applications. Similarly to other [DBUH]-based PILs, the probed ones behave as good ionic liquids. Molecular Dynamics-rationalised X-ray diffraction patterns show the major role played by hydrogen bonding in affecting morphology in these systems. Additionally we find further evidence of the existence of fluorous domains in [DBUH][IM14], thus potentially extending the range of applications for these PILs.

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Abstract: Non-graphitic carbons (NGCs) represent the most abundant class of \(sp^2\)-hybridized carbon materials (coal, char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks possessing significant structural disorder both in the single graphene sheets and the stacking. In this study an advanced evaluation approach for the wide-angle neutron scattering (WANS) was developed, based on the method introduced by Ruland and Smarsly (2002). In particular, we elucidated if and how the enhanced WANS data quality and larger values of the modulus of the scattering vector \(s\)-range affect the accuracy and the values of the size and disorder parameters - being fitting parameters by themselves - in comparison to wide-angle x-ray scattering (WAXS), which is usually performed by laboratory equipment. We find a reasonable agreement for the parameters \(L_\mathrm{a}\) and \(L_\mathrm{c}\), i.e. the lateral dimension and stack height, within the error bars, while for the disorder parameters different results for WAXS and WANS were found, the origin of which is discussed. Thus, this study addresses the general issue of how reliably microstructural parameters can be determined from WAXS/WANS, by fitting simulated WAXS and WANS curves, which are quality-impaired by added Gaussion noise at different levels and cut-off at different \(s\)-values. From this analysis we estimated the minimal data quality required for a reliable NGC microstructural analysis based on WAXS/WANS. As an important finding, these simulations show that typical, standard WAXS laboratory setups are sufficient to provide reliable values for the most relevant structural parameters. Furthermore, pair-distribution function (PDF) analyses were performed on WAXS data obtained from a Synchrotron facility. Comparing PDF and WAXS/WANS fitting analysis thus suggests the presence of small highly ordered oligoaromatic domains embedded in the larger graphene sheets, questioning the classical view on the NGC microstructure.

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Abstract: Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I–V–VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1–xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure–transport correlations of the thermoelectric AgBiSe2.

Open Access

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Abstract: The Diamond Light Source (DLS) beamline I15-1 measures atomic pair distribution functions (PDF) using scattering of 40-80 keV X-rays. A unique focusing element was needed to condense these X-rays from an initial large cross section (11.0 mm H × 4.2 mm V) into a required spot size of FWHM ≈680 μm (H) × 20 μm (V) at a variable position between the sample and the detector. The large numerical aperture is achieved by coating a silicon substrate over 1 m long with three multilayer stripes of Bragg angle 4.2 mrad. One stripe selects X-rays of each energy 40.0, 65.4, and 76.6 keV. Sixteen piezoelectric bimorph actuators attached to the sides of the mirror substrate adjusted the reflecting surface’s shape. Focal spots of vertical width < 15 µm were obtained at three positions over a 0.92 m range, with fast, easy switching from one focal position to another. Minimized root mean square slope errors were close to 0.5 µrad after subtraction of a uniform curvature. Reflectivity curves taken along each stripe showed consistent high peaks with generally small angular variation of peak positions. This is the first application of a 1 m long multilayer-coated bimorph mirror at a synchrotron beamline. Data collected with its help on a slice of a lithium ion battery’s cathode are presented.

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Abstract: The recent introduction of organometal halide perovskites to solar cells has significantly enhanced the power conversion efficiency of alternative photovoltaic devices, revolutionizing the development of photovoltaic technologies. To produce perovskite thin films with high device performances, various fabrication methodologies have been developed leading to thin films with different surface structures and crystal morphologies. Tremendous efforts have been devoted to characterizing macro- and microscopic structures within these films to better understand the processing-property-performance relationship. However, their atomic structure and its influence on device performance remains poorly understood. To this end, we employed pair distribution function analysis of X-ray total scattering data to obtain crystallographic and compositional information of methylammonium-lead-iodide (MAPbI3) thin films. This analysis revealed the ubiquitous presence of two near-amorphous intermediate phases with local structures that share subtle but significant correlations with the PbI2 precursor and the desired perovskite phase. The structure transformation from these intermediates to the perovskite deviates from the intuitive belief where the molecular cations get inserted between the sheets of layered PbI2 upon the crystallization of perovskite. This knowledge offers critical insight into the perovskite formation thermodynamics and reveals an important link between the short-range structure of the thin films and their corresponding device performance.