Hybrid systems consisting of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) are promising candidates for future optoelectronic device architectures, as they combine the outstanding charge transport properties of TMDCs with the high photoabsorption cross-section and the ability to tailor the energy levels of OSCs through synthetic means. Since crystalline organic layers have very anisotropic optical absorption and emission characteristics, their azimuthal alignment is an important parameter for the efficiency of the optical coupling in such hybrid systems. The lack of dangling bonds makes surfaces of 2D materials quite inert and smooth and therefore allows an unrestricted growth of organic adlayers without the requirement of relaxation or commensurability at the interface, which is an important prerequisite for van der Waals (vdW) epitaxy. Here, we have studied the formation and azimuthal alignment of crystalline adlayers of the OSC perfluoropentacene (PFP) on the basal plane of MoSe2, WSe2, MoS2, and MoTe2 single crystals representing the most ideal TMDC surfaces, which are further compared with corresponding films on graphene and hBN. Combining specular X-ray reflectivity with polarization resolved optical reflection measurements allowed the azimuthal alignment of crystalline PFP domains to be precisely analyzed, which revealed characteristic twist angles between adlayer and substrate lattices for the various 2D materials. This orientational order can be rationalized as an on-line coincidence and is well reproduced by model calculations that are based on the scheme of projection of real-space adlayer lattice points onto the substrate unit cell. The extreme sensitivity of the resulting rotational alignment of epitaxial adlayers on the smallest changes of the lattice parameters is demonstrated by distinctly different twist angles for PFP films that were grown at an elevated temperature and thus show a slight thermal expansion. The presently introduced and validated model to describe the epitaxial alignment in weakly bound crystalline adlayers can also be applied to other vdW bound heterosystems and can be useful to elucidate the peculiarities of vdW epitaxy.

The eightfold 2a0×2b0×2c0 superstructures of 6R‐CuxTa1+yS2 are reported for crystal A with x = 0.237 and y = 0, and crystal B with x = 0.23 and y = 0.06. Compelling evidence is presented for the self‐intercalation of tantalum onto octahedral sites, based on the diffraction data and crystal chemical arguments. The eightfold superstructure is formed by partial vacancy ordering within planes of intercalated atoms, while the stacking of TaS2 layers remains that of the 6R polytype. Temperature‐dependent X‐ray diffraction experiments show that the superstructure exists between T = 14 K and at least 370 K. Intercalated atoms enter the compound at octahedral and tetrahedral sites in alternating Van der Waals gaps. Intercalation is inhomogeneous, with average occupancies varying between 0.11 for octahedral sites within one Van der Waals gap and 0.23 for the tetrahedral sites within another Van der Waals gap. It is proposed that partial vacancy ordering is governed by the principle of maximum separation between intercalated atoms.

We investigated the compound 1T-Cu Ta S with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu Ta S resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS in which tantalum is partially substituted by copper. 1T-Cu Ta S readily decomposes at elevated temperatures and can only be prepared and stabilized by a sufficiently high amount of sulfur excess. XPS and NEXAFS measurements reveal that copper has the oxidation state  +I in 1T-Cu Ta S, which is supported by quantum chemical calculations. The disorder introduced by copper doping causes an Anderson-type localization of the conduction electrons as manifested by a strong increase of the electrical resistivity and a Curie-type paramagnetism at low temperatures as in other doped systems 1T-M Ta S with higher valent metals. Quantum chemical calculations support this interpretation.

The mixed conductor La 2 NiO 4+δ is an attractive candidate for SOFC cathodes and oxygen separation membranes. For this purpose long term stability is required. Its structural as well as defect and transport properties have been extensively studied during the last decades but are still not fully understood. La 2 NiO 4+δ is the first member of the Ruddlesden-Popper phases La 2n+1 Ni n O 3n+1 where n=1, 2, 3,… determines the stacking sequence along the c-axis which consists of n perovskite layers between any two rock-salt layers. The rock salt layer accommodates oxygen interstitial ions, providing oxygen excess δ up to 0.25 [1,2]. The aim of this study is to understand the anisotropic morphological and structural change of the near-surface region of a single crystal of La 2 NiO 4+δ that we observed above 1273K.[3] These structural and morphological changes were studied using SEM as well as TEM (HAADF & BF) Tantalum forms a series of polytypic sulfides of general composition Cu y Ta 1+x S 2 . [1,2] The layered-type phases are good electronic and ionic conductors. The mobile Cu ions (y) and excess Ta (x) are inserted in spatially separated van der Waals gaps of a layered TaS 2 host structure. Oxidative deintercalation of Cu with iodine affords new polymorphs of Ta 1+x S 2 .They differ from known polymorphs accessible from the elements at temperatures above 1250 K by higher order staging, i.e. excess Ta (x) does accumulate in part of the van der Waals gaps thereby conferring lubricant-like properties to theses Ta-rich disulfides. The polymorphs can be classified by a code nN; n counts the number of TaS 2 slabs in the repeat unit and N refers to the lattice symmetry, i.e., H for hexagonal, T for trigonal, R for rhombohedral. The structure of a ternary 4H polytype is presented which upon progressive deintercalation of Cu topotactically transforms into a differing 4H polymorph. The transition is reflected in the divergent X-ray diffractograms showing an offset of the reflection and a drastic change in the intensity modulation of the reflections of the h0l zone. Structural features of the two polymorphs derived from the distinctive intensity modulations are high-lighted and discussed in the light of known polymorphs. Electronic and mechanistic aspects of the transformation are discussed.[1] Harbrecht B., Kreiner G., Z. anorg. Allg. Chem., 572 (1989) 2 , platinum(II) is positioned at a centre of inversion with a pseudo-square planar coordination geometry with Pt-S distances in the range 2.284(13) -2.332(2) Å at 202 K and 2.305(12) -2.327(5) at 295 K. The ligand tetrahydrotiophene possesses a conformational flexibility, manifested by a dynamic disorder. There is a phase change at 201(1) K with the b-axis doubled and a third phase with the b-axis tripled is observe at 150 and 100 K. The checkCIF/PLATON service (IUCr) gives eight Clevel alerts at 295 K which are reduced to four at 202 K. The room temperature phase has a c-axis showing negative expansion behaviour. Thermal expansion coefficients as well as diagonalised...