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, Mossbauer 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 R (3) over barm 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) mu B, respectively. Furthermore, FeAl2Se4 enters a frozen spin-disordered state below 12 K.