The electronic structure of the alloyed transition-metal phosphorus trichalcogenide van der Waals Fe1–x Ni x PS3 compounds is studied using X-ray absorption spectroscopy and resonant photoelectron spectroscopy combined with intensive density functional theory calculations. Our systematic spectroscopic and theoretical data demonstrate the strong localization of the Fe- and Ni-ions-derived electronic states that leads to the description of the spectroscopic data as belonging simultaneously to Mott–Hubbard and charge-transfer insulators. These findings reveal Fe1–x Ni x PS3 as unique layered compounds with dual character of the insulating state, pointing to the importance of these results for the description and understanding of the functionality of this class of materials in different applications.

Large-scale high-quality van der Waals CoPS 3 single crystals are synthesized using a chemical vapor transport (CVT) method. The crystallographic structure and electronic properties of this layered material are systematically studied using different spectroscopic methods (XPS, NEXAFS, and resonant photoelectron spectroscopy) accompanied by density functional theory (DFT) calculations. All experimental and theoretical data allow assignment of this material to the class of mixed Mott−Hubbard/charge-transfer insulator with U dd ≅ Δ. All obtained results can enrich the information on the new class of van der Waals materials, transition metal phosphorus trichalcogenides, and help to further effectively exploit their electronic, optical, and transport properties, which are important for adopting this kind of materials into different application areas, such as spintronics and catalysis.

The stability and electronic structure of the possible Janus phase for the representative example of transition metal trichalcogenide FePS 1.5 Se 1.5 crystals are studied and discussed. The respective stoichiometric layered parent (FePS 3 and FePSe 3 ) and mixed (FePS 1.5 Se 1.5 ) compounds are successfully synthesized, and our experimental data confirm the high quality of the obtained crystals and the uniform random distribution of S and Se atoms in the chalcogens' layers for the FePS 1.5 Se 1.5 crystals. These results are supported by the detailed density functional theory calculations, which indicate the absence of the Janus phase for the layered 3D FePS 1.5 Se 1.5 . This effect is explained by the large uncompensated dipole moment for the Janus phase, and the possible routes for the stabilization of 2D FePS 1.5 Se 1.5 Janus layers are discussed.

The interaction of high-quality transition metal trichalcogenides (TMTs) single crystals FePX3 (X: S, Se) with water molecules is studied using near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) in a wide range of temperature and partial pressure of H2O. The physisorption nature of interaction between H2O and FePX3 is found at low temperatures and relatively small concentrations of water molecules, that is supported by the DFT results. When temperature of the FePX3 samples and partial pressure of H2O are increased, the interaction at the interface is defined by two competing processes -- adsorption of molecules at high partial pressure of H2O and desorption of molecules due to the increased surface mobility and physisorption nature of interaction. Our intensive XPS/NEXAFS experiments accompanied by DFT calculations bring new understanding on the interaction of H2O with surface of a new class of 2D materials, TMTs, pointing to their stability and reactivity, that is important for further applications in different areas, like sensing and catalysis.

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