Pressure-dependent excitonic instability and structural phase transition in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$: Raman and first-principles study

${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, a semiconductor at ambient conditions, does not exhibit an excitonic insulating state like its selenium counterpart ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, 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 ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ with pressure. We observe pressure-induced structural phase transition from its orthorhombic $Cmcm$ structure to another orthorhombic $Pmnm$ structure, with onset at $\sim$ 4.2 GPa and this transition gets completed at $\sim$ 6 GPa. We observe Raman signatures of an additional phase transition at $\sim$ 10.8 GPa, which is suggested to be associated with a semiconductor to metal transition.