Ferromagnetic van der Waals Crystal VI3.

We report structural properties, physical properties, and the electronic structure of van der Waals (vdW) crystal VI3. Detailed analysis reveals that VI3 exhibits a structural transition from monoclinic C2/ m to rhombohedral R3̅ at Ts ≈ 79 K, similar to CrX3 (X = Cl, Br, I). Below Ts, a long-range ferromagnetic (FM) transition emerges at Tc ≈ 50 K. The local moment of V in VI3 is close to the high-spin state V3+ ion ( S = 1). Theoretical calculations suggest that VI3 may be a Mott insulator with a band gap of about 0.90 eV. In addition, VI3 has a relatively small interlayer binding energy and can be exfoliated easily down to a few layers experimentally. Therefore, VI3 is a candidate for two-dimensional FM semiconductors. It also provides a novel platform to explore 2D magnetism and vdW heterostructures in S = 1 system.