Origin of the Insulating Phase and First-Order Metal-Insulator Transition in 1T-TaS_{2}.

Using density functional theory calculations, we investigate the origin of the insulating phase and metal-insulator transition (MIT) in octahedral tantalum disulfide (1T-TaS_{2}), a layered van der Waals material with a prominent two-dimensional (2D) charge density wave (CDW) order. We show that the MIT is driven not by the 2D order itself, but by the vertical ordering of the 2D CDWs or the 3D CDW order. We identify two exceptionally stable 3D CDW configurations; one is insulating and the other is metallic. The competition and mixing of the two CDW configurations account for many mysterious features of the MIT in 1T-TaS_{2}, including the pressure- and doping-induced transitions and the hysteresis behavior. The present results emphasize that interlayer electronic ordering can play an important role in electronic phase transitions in layered materials.