Comparison of the Electronic Structures of V$X_3$ ($X$ = Br, and I) using High-resolution X-ray Scattering

TL;DR

The paper addresses the uncertain ground-state electronic structure of vanadium trihalides VX$_3$ (X = Br, I) by combining high-resolution RIXS with ligand-field multiplet theory. The authors determine crystal-field splitting ($10Dq$), trigonal distortion ($\Delta_{D_{3d}}$), Racah parameters, and Coulomb/uplift interactions, revealing a high-spin $V^{3+}$ ($S=1$) state with $e'_g{}^2$ occupancy in VBr$_3$ and $e'_g a_{1g}^1$ in VI$_3$, driven by opposite signs of $\Delta_{D_{3d}}$. The results show increasing covalency from Br to I and demonstrate how SOC and ligand-field tuning govern the low-energy electronic structure, with implications for 2D magnetism and spintronics. The work provides a robust experimental foundation for modeling VX$_3$ and informs material design strategies for tunable electronic and magnetic properties in layered vanadium halides.

Abstract

Transition-metal halides V$X_3$ ($X$ = Br and I) have emerged as promising candidates for two dimensional spintronic and quantum applications due to their layer-dependent magnetism and tunable electronic states. However, experimental insights into their ground state electronic structures remain limited. Here, we present a comprehensive investigation of V$X_3$ using high resolution resonant inelastic x-ray scattering (RIXS) combined with ligand field multiplet calculations. The RIXS spectra reveal distinct $dd$ and charge-transfer excitations, allowing precise determination of electronic structure parameters, including the crystal field splitting, trigonal distortion, and Racah parameters. The determined parameters showed clear variation, indicating an increase in covalency from Br to I. The trigonal distortion parameters $Δ_{D_{3d}}$ were determined to be -0.096 eV in VBr$_3$ and 0.07 eV in VI$_3$, indicating a sign opposition between the two compounds, reflecting good agreement with experimental RIXS data. Cluster model calculations yield a high-spin V$^{3+}$ $(S = 1)$ configuration, with an $e'^2_g$ ground state in VBr$_3$ and an $e'^1_ga^1_{1g}$ ground state in VI$_3$, consistent with trigonal elongation and compression, respectively. Our findings provide the most complete experimental determination of the low energy electronic structure in V$X_3$, offering valuable insights for designing 2D magnetic and spintronic materials based on vanadium halides.

Figures (8)

• Figure 1: (a) crystal structure of V$X_3$ in monoclinic and (b) rhombohedral structure (c) Crystal field splitting of the V $3d$ orbitals when the symmetry is lowered from $O_h$ to $D_{3d}$ symmetry. The Type I (II) configuration corresponds to the trigonal compression in V$X_3$ as shown in the V$X_6$ cluster unit.
• Figure 2: V $L$ edge XAS data in VBr$_3$ and VI$_3$ at different x-ray incident geometries. The blue, red, and green lines indicate the XAS data collected at $20^0$, $50^0$, and $90^0$ with respect to the sample surface (see the top right schematic diagram). The yellow shaded spectra show the simulated XAS spectra at 300 K.
• Figure 3: Temperature dependent V $L-$ edge XAS data in VBr$_3$. The purple, yellow, and blue lines show the data collected at 14, 65, and 300 K, respectively. All the data were measured at the normal incidence geometry.
• Figure 4: a) Temperature dependence RIXS measurements in VBr$_3$ and b) VI$_3$ at normal incidence. The regions I, II, III, and IV are labeled to show different spectral features in V$X_3$.
• Figure 5: a) Excitation energy dependence RIXS measurements in VBr$_3$ and b) VI$_3$ at 300 K. The solid and dashed lines indicate the normal incidence and grazing incidence, respectively. The excitation energies a, b, c, and d are as labeled in Figure \ref{['fig:XAS_VX3_angular_dependency']}