Polaronic and Mott insulating phase of layered magnetic vanadium trihalide ${\mathrm{VCl}}_3$

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 $e_g^{\prime }$ 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.

Figure 1

(a) Top and side views of ${\mathrm{VCl}}_3$ monolayer crystal structure. V and Cl atoms are represented in red and green spheres, respectively. (b) SEM image of ${\mathrm{VCl}}_3$ flake-like crystals. (c) Room-temperature XRD experimental (black crosses) and $R\overline{3}$-model fitted (red line) spectra of ${\mathrm{VCl}}_3$ crystal. (d) Temperature-dependent heat capacity of ${\mathrm{VCl}}_3$ crystal. Insets: the antiferrogmanet phase transition at 21.8 K and the structural phase transition at 103 K.

Figure 2

XPS core levels and valence bands spectra of as-exfoliated ${\mathrm{VCl}}_3$ flakes. The raw data (black), the cumulative fits (dark green), and the relative deconvolution (colored) are shown. (a) O-$1s$ and V-$2p$ core levels deconvolution. (b) Cl-$2p$ core level deconvolution. (c) The experimental valence bands spectrum of ${\mathrm{VCl}}_3$ flake (black) is directly compared to the DOS of pristine ${\mathrm{VCl}}_3$ monolayer (red). The DOS is convoluted with a Gaussian function ($\sigma =0.5$ eV) to account for the experimental broadening.

Figure 3

Spin-resolved band structures of pristine ${\mathrm{VCl}}_3$ monolayer.

Figure 4

Electron small 2D polarons of ${\mathrm{VCl}}_3$. (a) Spin-resolved partial DOS of polaronic $2\times 2$ supercell of ${\mathrm{VCl}}_3$. Polaronic states are highlighted and contributed mainly by V-$3d$ states. (b) The experimental valence bands spectrum of ${\mathrm{VCl}}_3$ flake (black) is directly compared to the DOS of polaronic monolayer configurations (violet), convoluted with a Gaussian function ($\sigma =0.5$ eV) to account for the experimental broadening. (c) Polaron charge density on the ${\mathrm{VCl}}_{3}2\times 2$ supercell. V and Cl atoms are represented in red and green spheres, respectively. (d) Electrons occupation for ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{2+}$ polaronic configurations. (e) Spatially-projected DOS of V-$3d$ states for ${\mathrm{V}}^{2+}$ polaronic atom and ${\mathrm{V}}^{3+}$ atom.

Figure 5

Projected DOS for V-$3d$ states for ${\mathrm{V}}^{2+}$ polaronic atom (blue) and ${\mathrm{V}}^{3+}$ atom (dashed blue) of ${\mathrm{VI}}_3$ monolayer are compared with a ${\mathrm{VI}}_3$ photoemission spectrum in resonance with V-$3d$ states (red), which is adapted from Fig. 4(a) of Ref. [6].