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Magnetic, Structural, and Electronic Properties of CrOCl with the PBE Functional

Brahim Marfoua, Mohammad Amirabbasi, Marcus Ekholm

TL;DR

This work shows that spin-polarized PBE without on-site corrections correctly stabilizes the AFM ground state of CrOCl and, when combined with a van der Waals dispersion term, accurately reproduces structural parameters. Hybrid functionals and DFT+$U$ schemes, particularly using the Dudarev parametrization, can erroneously favor FM ordering or overestimate volumes, underscoring that correlation effects in CrOCl are not as strong as often assumed. Electronic-structure analysis reveals Cr-dominated valence bands with an insulating gap that arises from band structure rather than Mott physics, with orbital splitting consistent across functionals and a notable role for direct exchange and ligand hybridization. These findings establish PBE as a robust, parameter-free starting point for CrOCl-based materials and provide insights into the exchange mechanisms driving AFM order, informing future theoretical and experimental studies of low-dimensional CrOCl systems.

Abstract

CrOCl is a van der Waals-layered insulator with an antiferromagnetic ground state, making it a promising platform for exfoliation and the exploration of low-dimensional magnetism. An accurate ab initio description is therefore essential. Previous density-functional studies have shown that DFT+$U$ calculations may erroneously favor ferromagnetic order depending on the choice of parametrization, an issue that cannot be remedied by simply adjusting the value of $U$. Here, we demonstrate that an explicit Hubbard correction is unnecessary: the PBE functional correctly reproduces the AFM ground state while simultaneously improving the description of structural properties. Moreover, PBE provides a reliable account of the electronic structure. These findings clarify the role of correlation effects in CrOCl and identify PBE as a robust starting point for future ab initio studies of CrOCl-based materials.

Magnetic, Structural, and Electronic Properties of CrOCl with the PBE Functional

TL;DR

This work shows that spin-polarized PBE without on-site corrections correctly stabilizes the AFM ground state of CrOCl and, when combined with a van der Waals dispersion term, accurately reproduces structural parameters. Hybrid functionals and DFT+ schemes, particularly using the Dudarev parametrization, can erroneously favor FM ordering or overestimate volumes, underscoring that correlation effects in CrOCl are not as strong as often assumed. Electronic-structure analysis reveals Cr-dominated valence bands with an insulating gap that arises from band structure rather than Mott physics, with orbital splitting consistent across functionals and a notable role for direct exchange and ligand hybridization. These findings establish PBE as a robust, parameter-free starting point for CrOCl-based materials and provide insights into the exchange mechanisms driving AFM order, informing future theoretical and experimental studies of low-dimensional CrOCl systems.

Abstract

CrOCl is a van der Waals-layered insulator with an antiferromagnetic ground state, making it a promising platform for exfoliation and the exploration of low-dimensional magnetism. An accurate ab initio description is therefore essential. Previous density-functional studies have shown that DFT+ calculations may erroneously favor ferromagnetic order depending on the choice of parametrization, an issue that cannot be remedied by simply adjusting the value of . Here, we demonstrate that an explicit Hubbard correction is unnecessary: the PBE functional correctly reproduces the AFM ground state while simultaneously improving the description of structural properties. Moreover, PBE provides a reliable account of the electronic structure. These findings clarify the role of correlation effects in CrOCl and identify PBE as a robust starting point for future ab initio studies of CrOCl-based materials.

Paper Structure

This paper contains 11 sections, 5 equations, 6 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Computational Details
  3. Results and Discussion
  4. Magnetic ground state
  5. Fixed lattice calculations
  6. Structural Optimization
  7. Electronic Structure
  8. Density of states
  9. Orbital splitting
  10. Exchange Interactions
  11. Summary and Conclusions

Figures (6)

  • Figure 1: Structural illustration with (a) the top view ($ab$-plane) and (b) side view ($bc$- and $ac$-planes) of the orthorhombic CrOCl bulk system. Chromium (Cr) atoms are represented in blue, chlorine (Cl) atoms in green, and oxygen (O) atoms in red. Figures. (c)-(e) show various magnetic configurations represented by upward and downward arrows on the Cr magnetic atoms.
  • Figure 2: Energy differences (meV / unit cell) between the FM and experimental AFM configurations with various functionals for orthorhombic CrOCl with experimental lattice coordinates. Positive values indicate the FM to be favored, while negative values indicate the AFM state is favored.
  • Figure 3: Energy versus volume curves for antiferromagnetic (AFM) and ferromagnetic (FM) configurations obtained from (a) PBE$+U$, (b) PBE$+U$+$J$, and (c) PBE functionals. The equation of state (EoS) fits for the FM (dashed line) and AFM (solid line) configurations are fitted based on data points, with squares representing FM and circles representing AFM configurations. The energy is scaled to the minimum energy of the AFM structure. The horizontal dashed gray line indicates the zero energy reference, and the vertical dashed black line marks the experimental volume.
  • Figure 4: Site resolved density of states (DOS) for Cr, O, and Cl in various functionals: (a) LDA, (b) PBE, (c) PBE$+U$ ($U=3.0$ eV), (d) PBE$+U$+$J$ ($U=3.0$ eV and $J=1.5$ eV), (e) SCAN, and (f) HSE06. The solid blue line represents the DOS for Cr, the dashed red line for O, and the dot-dashed green line for Cl.
  • Figure 5: (a) Distorted CrO$_4$Cl$_2$ octahedron with local $(x,y,z)$ coordinate frame, with $x$ along $\mathbf{b}$, $y$ along $\mathbf{c}$ and $z$ along $\mathbf{a}$. Spin resolved projected DOS obtained with (b) the PBE functional and (c) the PBE$+U$+$J$ functional.
  • ...and 1 more figures