Ferroelectricity, spin-singlet formation and orbital-selective Peierls phase in the dihalide MOX₂ family with d¹ and d² configurations

The van der Waals (vdW) family of layered oxide dihalide MOX₂ (M = V, Ta, Nb, Os; X = halogen element) display an interesting geometric structure. Here, we systematically study VOI₂ and MoOCl₂ with 3d¹ and 4d² electronic configurations, respectively. In VOI₂, we found the “pseudo-Jahn-Teller” effect caused by the coupling between empty V (d_xz/yz and d_3z²−r²) and O 2p states. We proposed this as the mechanism that stabilizes the ferroelectric 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 ^[1]. In MoOCl₂, our main result is that an orbital-selective Peierls phase develops in MoOCl₂, resulting in the dimerization of the Mo chain along the b-axis. Specifically, the Mo-d_xy orbitals form robust molecular-orbital states inducing localized d_xy singlet dimers, while the Mo-d_xz/yz orbitals remain delocalized and itinerant. Our study shows that MoOCl₂ is globally metallic, with the Mo-d_xy orbital bonding-antibonding splitting, opening a gap and the Mo-d_xz/yz orbitals contributing to the metallic conductivity ^[2].