Mn₂Ga₂S₅ and Mn₂Al₂Se₅ van der Waals Chalcogenides: A Source of Atomically Thin Nanomaterials

Layered chalcogenides containing 3d transition metals are promising for the development of two-dimensional nanomaterials with interesting magnetic properties. Both mechanical and solution-based exfoliation of atomically thin layers is possible due to the low-energy van der Waals bonds. In this paper, we present the synthesis and crystal structures of the Mn₂Ga₂S₅ and Mn₂Al₂Se₅ layered chalcogenides. For Mn₂Ga₂S₅, we report magnetic properties, as well as the exfoliation of nanofilms and nanoscrolls. The synthesis of both polycrystalline phases and single crystals is described, and their chemical stability in air is studied. Crystal structures are probed via powder X-ray diffraction and high-resolution transmission electron microscopy. The new compound Mn₂Al₂Se₅ is isomorphous with Mn₂Ga₂S₅ crystallizing in the Mg₂Al₂Se₅ structure type. The crystal structure is built by the ABCBCA sequence of hexagonal close-packing layers of chalcogen atoms, where Mn²⁺ and Al³⁺/Ga³⁺ species preferentially occupy octahedral and tetrahedral voids, respectively. Mn₂Ga₂S₅ exhibits an antiferromagnetic-like transition at 13 K accompanied by the ferromagnetic hysteresis of magnetization. Significant frustration of the magnetic system may yield spin-glass behavior at low temperatures. The exfoliation of Mn₂Ga₂S₅ layers was performed in a non-polar solvent. Nanolayers and nanoscrolls were observed using high-resolution transmission electron microscopy. Fragments of micron-sized crystallites with a thickness of 70-100 nanometers were deposited on a glass surface, as evidenced by atomic force microscopy.