Breakdown of electron lone pair and insulator-⁠to-⁠metal transition in MS₃ (M=Ti,Zr,Hf) under high-⁠pressure

Layered transition metal trisulfides (MS₃, M=Ti,Zr,Hf) have gained significant attention for their remarkable anisotropic optical and electronic properties, presenting immense potential for various applications. At ambient pressure, these materials crystallize in a monoclinic structure (P2₁/m) and function as n-type semiconductors with band gaps ranging from 1–2 eV. Experimental studies report that TiS3 undergoes a semiconductor-to-metal transition (MIT) near 22 GPa, accompanied by an iso-structural phase transition. Similarly, ZrS3 and HfS3 exhibit iso-structural transitions at 17 GPa and 18 GPa, respectively, though their MITs occur at higher pressures. However, the electronic origin of these transitions is not well understood.

Using density functional theory (DFT) and orbital-based bonding descriptors, we reveal that TiS₃ develops multicenter interactions under pressure due to charge transfer from the 3s² lone electron pair (LEP) to the ppσ*-antibonding state of S-S dimers, causing structural instability. The iso-structural transition is driven by the interchange of primary and secondary S-S bonds, while metallization arises from reduced charge transfer from Ti to bridging S atoms. For ZrS₃ and HfS₃, our calculations confirm that their structural transitions are also governed by S-S bond rearrangements and structural transtion within the same crystal symmetry is not responsible for metallization.