Unraveling Anionic Charge Disproportionation, Charge Density Wave and Their Anomalous Interplay in 1T-IrS₂

Understanding the interplay between charge density wave (CDW) and charge disproportionation (CD) is essential for unraveling the electronic behavior and phase transition mechanisms of materials. Typically, a CDW phase transition introduces inequivalent cation sites, resulting in a CD state. Here, the presence of an in-plane 2 × 1 CDW in monolayer 1T-IrS₂​, as well as in its bilayer and bulk phases with traditional AA stacking is reported here for the first time. However, the CDW does not induce a CD state, as it is characterized by the complete in-plane dimerization of equivalent Ir⁴⁺ ions. Remarkably, when the stacking order of 1T-IrS₂ is altered, a CD state featuring interlayer S-S dimerization emerges, leading to a metal-insulator transition and suppressing the CDW instability. It is further revealed that the CD state arises from asymmetric Ir-S charge transfer induced by S-S dimerization, and thus exhibits Ir³⁺, S¹⁻, and S²⁻ effective valence states. Therefore, the CD state in 1T-IrS₂ represents a unique anionic CD, in contrast to the traditional cationic CD. This work underscores the critical role of stacking order in uncovering novel quantum phases in layered materials and provides new insights into the correlation between CD and CDW states.