Phase transition in van der Waals (vdW) two-dimensional (2D) materials is an effective approach to create novel and tunable electronic states for potential applications in next-generation quantum electronics. Here, we propose an unusual non vdW phase transition behavior in 2D layered transition metal dichalcogenides (TMDs). As interlayer distance decreases below the common vdW gap upon pressure, interfacial ligand-ligand chemical bonds could form to stabilize the system and result in a phase transition to a non vdW structure, which exhibits interesting electronic and magnetic properties very different from the pristine vdW one. Based on first-principles calculations, we predict stable non vdW phase transitions in $\mathrm{Cr}{\mathrm{S}}_2$ and other TMD bilayers, characterized by the formation of interlayer S-S single covalent bonds, which is accompanied by metal-semiconductor, nonmagnetic-magnetic and/or magnetic phase transitions. These findings elucidate a general phase-transition mechanism in vdW 2D materials driven by out-of-plane pressure, suggesting a viable strategy to exploring potential quantum electronic states for diverse functional applications.

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• Phase transitions
• 2-dimensional systems
• Ferrimagnets
• Magnetic semiconductors
• Transition metal dichalcogenides
• First-principles calculations