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

Abstract

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