Layered van der Waals Chalcogenides FeAl2Se4, MnAl2S4, and MnAl2Se4: Atomically Thin Triangular Arrangement of Transition-Metal Atoms

Layered van der Waals (vdW) chalcogenides of 3d transition metals are a rich source of two-dimensional (2D) nanomaterials, in which atomically thin layers with the terminating chalcogen atoms exhibit promising functionality for novel spintronic devices. Here, we report on the synthesis, crystal growth, and magnetic properties of FeAl₂Se₄, MnAl₂S₄, and MnAl₂Se₄ ternary chalcogenides. Crystal structures are probed by powder X-ray diffraction, Mössbauer spectroscopy, and high-resolution transmission electron microscopy. We improve the structural models of FeAl₂Se₄ and MnAl₂S₄ and show that isostructural MnAl₂S₄ and MnAl₂Se₄ crystallize in the centrosymmetric R3̅̅m space group. In the crystal structure, transition metal and Al atoms mutually occupy the octahedral and tetrahedral voids of four close-packing chalcogen layers terminated by vdW gaps. The transition-metal atoms form a triangular arrangement inside the close-packing layers. As a result, FeAl₂Se₄ and MnAl₂S₄ show no long-range magnetic order in the studied temperature range. In the paramagnetic state, Fe and Mn possess effective magnetic moments of 4.99(2) and 5.405(6) μ_B, respectively. Furthermore, FeAl₂Se₄ enters a frozen spin-disordered state below 12 K.