Using ab initio density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer ${\mathrm{VOI}}_2$ with a $3d^1$ electronic configuration. Our phonon calculations indicate that the orthorhombic $Pmm2$ FE-II phase is the most likely ground state, involving a ferroelectric (FE) distortion along the $a$ axis and V-V dimerization along the $b$ axis. Specifically, the “pseudo Jahn-Teller” effect caused by the coupling between empty V ($d_{xz/yz}$ and $d_{3z^2-r^2}$) and O $2p$ states is proposed as the mechanism that stabilizes the FE distortion from the paraelectric phase. Moreover, the half-filled metallic $d_{xy}$ band displays a Peierls instability along the $b$ axis, inducing a V-V dimerization. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state.

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• Ferroelectricity
• First-principles calculations
• Peierls transition
• Phase transitions
• 1-dimensional spin chains
• Ferroelectrics
• Monolayer films
• Density functional calculations
• Density matrix renormalization group
• Hubbard model