X-ray electron density investigation of chemical bonding in van der Waals materials.

Van der Waals (vdW) solids have attracted great attention ever since the discovery of graphene, with the essential feature being the weak chemical bonding across the vdW gap. The nature of these weak interactions is decisive for many extraordinary properties, but it is a strong challenge for current theory to accurately model long-range electron correlations. Here we use synchrotron X-ray diffraction data to precisely determine the electron density in the archetypal vdW solid, TiS2, and compare the results with density functional theory calculations. Quantitative agreement is observed for the chemical bonding description in the covalent TiS2 slabs, but significant differences are identified for the interactions across the gap, with experiment revealing more electron deformation than theory. The present data provide an experimental benchmark for testing theoretical models of weak chemical bonding.

The structure-directing amine changes everything: structures and optical properties of two-dimensional thiostannates.

Two different two-dimensional thiostannates (SnS) were synthesized using tris(2-aminoethyl)amine (tren) or 1-(2-aminoethyl)piperidine (1AEP) as structure-directing agents. Both structures consist of negatively charged thiostannate layers with charge stabilizing cations sandwiched in-between. The fundamental building units are Sn3S4 broken-cube clusters connected by double sulfur bridges to form polymeric (Sn3S72-)n honeycomb hexagonal layers. The compounds are members of the R-SnS-1 family of structures, where R indicates the type of cation. Despite consisting of identical structural units, the band gaps of the two semiconducting compounds were found to differ substantially at 2.96 eV (violet-blue light) and 3.21 eV (UV light) for tren-SnS-1 and 1AEP-SnS-1, respectively. Aiming to explain the observed differences in optical properties, the structures of the two thiostannates were investigated in detail based on combined X-ray diffraction, solid-state 13C and 119Sn MAS NMR spectroscopy and scanning electron microscopy studies. The compound tren-SnS-1 has a hexagonal structure consisting of planar SnS layers with regular hexagonal pores and disordered cations, whereas 1AEP-SnS-1 has an orthorhombic unit cell with ordered cations, distorted hexagonal pores and non-planar SnS layers. In the formation of 1AEP-SnS-1, an intramolecular reaction of the structure-directing piperidine takes place to form an N-heterobicyclic cation through in situ C-H activation. Hirshfeld surface analysis was used to investigate the interaction between the SnS layers and cations in 1AEP-SnS-1 and revealed that the most nucleophilic part of the SnS sheets is one of the two crystallographically distinct double sulfur bridges.

Electron Density Analysis of the "O-O" Charge-Shift Bonding in Rubrene Endoperoxide.

Rubrene endoperoxide p-xylene (1) has been obtained in crystalline form from recrystallization and purification of the organic semiconductor, rubrene, and for the first time characterized by single-crystal X-ray diffraction methods. 1 is produced by reaction of rubrene with molecular oxygen to create rubrene endoperoxide, C42H28O2, in which an O2-bridge is joining the two phenyl-substituted C atoms opposite each other in the second of the four aromatic rings in tetracene thereby breaking the resonance along the tetracene moiety. The electron density distribution of 1 reveals that the intramolecular O-O bond is best characterized as charge-shift bonding with loss of electronic concentration in the interatomic region evidenced by the Laplacian. Likewise the ELI-D indicates little electron localization in this region. Furthermore, source function and ELI-D analysis of 1 clearly quantifies the lack of electronic delocalization across the six-membered ring that carries the peroxide-bridge.

Visualizing lithium-ion migration pathways in battery materials.

The understanding of lithium-ion migration through the bulk crystal structure is crucial in the search for novel battery materials with improved properties for lithium-ion conduction. In this paper, procrystal calculations are introduced as a fast, intuitive way of mapping possible migration pathways, and the method is applied to a broad range of lithium-containing materials, including the well-known battery cathode materials LiCoO2 , LiMn2 O4 , and LiFePO4 . The outcome is compared with both experimental and theoretical studies, as well as the bond valence site energy approach, and the results show that the method is not only a strong, qualitative visualization tool, but also provides a quantitative measure of electron-density thresholds for migration, which are correlated with theoretically obtained activation energies. In the future, the method may be used to guide experimental and theoretical research towards materials with potentially high ionic conductivity, reducing the time spent investigating nonpromising materials with advanced theoretical methods.