Layered van der Waals Chalcogenides FeAl 2 Se 4 , MnAl 2 S 4 , and MnAl 2 Se 4 : 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 FeAl 2 Se 4 , MnAl 2 S 4 , and MnAl 2 Se 4 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 FeAl 2 Se 4 and MnAl 2 S 4 and show that isostructural MnAl 2 S 4 and MnAl 2 Se 4 crystallize in the centrosymmetric R 3̅̅ m 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, FeAl 2 Se 4 and MnAl 2 S 4 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, FeAl 2 Se 4 enters a frozen spin-disordered state below 12 K.