Structure, dynamical stability, and electronic properties of phases in TaS2 from a high-level quantum mechanical calculation

TaS2 is a transition-metal dichalcogenide having an exceptionally rich phase diagram, which includes exotic phenomena such as a charge density wave. We analyzed the structure, bonding, ground state, and dynamical stability of 1T, 2H, and 3R phases of TaS2, and a commensurate charge density wave phase from the first principles. Van der Waals interaction among layers and strong electron-electron interactions were included by using the exact exchange plus random phase approximation, a high-level quantum mechanical approach. The calculated structural parameters agree well with the available experimental data. The individual sheets of TaS2 are bound by dispersive forces, which are stronger than dispersive forces in graphite and fluorographite. 1T-TaS2 is dynamically unstable at low temperature, which leads to the formation of charge density wave and opening of the in-plane band gap. Anharmonic phonon-phonon interactions stabilize the 1T structure at elevated temperatures. The calculated phase diagram of TaS2 reveals that the 1T phase is the ground state at temperatures above 1300 K, 2H below this point, and the charge density wave phase becomes more stable than the perfect 1T structure below 480 K.