Structure, dynamical stability, and electronic properties of phases inTaS2from a high-level quantum mechanical calculation

Abstract

${\mathrm{TaS}}_{2}$ 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 ${\mathrm{TaS}}_{2}$, 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 ${\mathrm{TaS}}_{2}$ are bound by dispersive forces, which are stronger than dispersive forces in graphite and fluorographite. $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ 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 ${\mathrm{TaS}}_{2}$ 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.