Tuning the phase transition of TaS2 polymorphs under high pressure and high temperature conditions

Abstract

The polymorphic transition in TaS₂ has demonstrated rich tunability in physical properties, including modulated charge density wave (CDW) orders and superconductivity (SC), which is crucial for the development of new-concept and functional devices. Although phase engineering of TaS₂ has been explored through alkali metal intercalation and strain engineering, achieving precise control over the transition among its polytypes remains highly desirable, and in-depth investigations into the physical mechanisms triggering these structural transitions are still limited. Here, we systematically explored the phase transition behaviors of TaS₂ polytypes (2H, 4Hb, 6R, and 1T phases) under high-pressure and high-temperature (HPHT) conditions. We constructed a detailed phase diagram of TaS₂ across a pressure range of 0–6 GPa and a temperature range of 800–2000 K. Our findings indicate that high pressure effectively destabilizes the T layer of TaS₂, while high temperature exerts the opposite effect. Theoretical calculations reveal that the interaction strength between the planar Ta–Ta atoms, modulated by HPHT conditions, is a critical factor driving the T-to-H transition. Specifically, variations in the electrostatic repulsion between the lone pair electrons of S atoms and interstitial electrons from Ta atoms effectively alter the bond angles of S–Ta–S in both T and H layers, leading to the distinct deviations from their ideal geometrical configuration. Our results suggest that the deviation degree of the bond angles of S–Ta–S serves as a reliable metric correlating with the preferred phase in the competition between 1T and 2H phases. Moreover, this principle extends to other transition metal dichalcogenides (TMDs) with varying d-electron numbers, from which we established volcano curves for their preferred phases based on the interactions among transition metal atoms. Our work not only provides a novel approach for modulating the phase preference of TaS₂ but also elucidates a general phase transition mechanism for tuning TMDs.