Pressure-dependent excitonic instability and structural phase transition in Ta2NiS5 : Raman and first-principles study

Ta₂NiS₅, a semiconductor at ambient conditions, does not exhibit an excitonic insulating state like its selenium counterpart Ta₂NiSe₅, owing to its large band gap. Using a combination of Raman spectroscopy and analysis with first-principles effective Hamiltonian, we explore its instability toward an excitonic insulating state as a function of pressure, and affirm that excitonic insulating state does not get stabilized in Ta₂NiS₅ with pressure. We observe pressure-induced structural phase transition from its orthorhombic $Cmcm$ structure to another orthorhombic $Pmnm$ structure, with onset at ~ 4.2 GPa and this transition gets completed at ~ 6 GPa. We observe Raman signatures of an additional phase transition at ~ 10.8 GPa, which is suggested to be associated with a semiconductor to metal transition.