Anomalous phonon Grüneisen parameters in semiconductor Ta_2NiS_5

Strain tuning is a powerful experimental method in probing correlated electron systems. Here we study the strain response of the lattice dynamics and electronic structure in semiconductor Ta_2NiS_5 by polarization-resolved Raman spectroscopy. We observe an increase of the size of the direct semiconducting band gap. Although the majority of the optical phonons show only marginal dependence to applied strain, the frequency of the two B_2g phonon modes, which have quadrupolar symmetry and already anomalously soften on cooling under zero strain, increases significantly with tensile strain along the a axis. The corresponding Grüneisen parameters are unusually large in magnitude and negative in sign. These effects are well captured by first-principles density functional theory calculations and indicate close proximity of Ta_2NiS_5 to a structural instability, similar to that encountered in excitonic insulator candidate Ta_2NiSe_5.