Polaronic and Mott insulating phase of layered magnetic vanadium trihalide VCl3

Two-dimensional (2D) van der Waals (vdW) magnetic $3d$-transition metal trihalides are a new class of functional materials showing exotic physical properties useful for spintronic and memory storage applications. In this article, we report the synthesis and electromagnetic characterization of single-crystalline vanadium trichloride, ${\mathrm{VCl}}_{3}$, a novel 2D layered vdW Mott insulator, which has a rhombohedral structure ($\mathrm{R}\overline{3}$, No. 148) at room temperature. ${\mathrm{VCl}}_{3}$ undergoes a structural phase transition at 103 K and a subsequent antiferromagnetic transition at 21.8 K. Combining core levels and valence bands x-ray photoemission spectroscopy (XPS) with first-principles density functional theory (DFT) calculations, we demonstrate the Mott Hubbard insulating nature of ${\mathrm{VCl}}_{3}$ and the existence of electron small 2D magnetic polarons localized on V atom sites by V-Cl bond relaxation. The polarons strongly affect the electromagnetic properties of ${\mathrm{VCl}}_{3}$ promoting the occupation of dispersion-less spin-polarized V-$3d {a}_{1g}$ states and band inversion with ${e}_{g}^{\ensuremath{'}}$ states. Within the polaronic scenario, it is possible to reconcile different experimental evidences on vanadium trihalides, suggesting that also ${\mathrm{VI}}_{3}$ hosts polarons. Our results highlight the complex physical behavior of this class of crystals determined by charge trapping, lattice distortions, correlation effects, mixed valence states, and magnetic states.