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The structural phase transition in Ta$_2$NiSe$_5$ has been envisioned asdriven by the formation of an excitonic insulating phase. However, the role ofstructural and electronic instabilities on crystal symmetry breaking has yet tobe disentangled. Meanwhile, the phase transition in its complementary materialTa$_2$NiS$_5$ does not show any experimental hints of an excitonic insulatingphase. We present a microscopic investigation of the electronic and phononiceffects involved in the structural phase transition in Ta$_2$NiSe$_5$ andTa$_2$NiS$_5$ using extensive first-principles calculations. In both materialsthe crystal symmetries are broken by phonon instabilities, which in turn leadto changes in the electronic bandstructure also observed in experiment. A totalenergy landscape analysis shows no tendency towards a purely electronicinstability and we find that a sizeable lattice distortion is needed to open abandgap. We conclude that an excitonic instability is not needed to explain thephase transition in both Ta$_2$NiSe$_5$ and Ta$_2$NiS$_5$.

Common microscopic origin of the phase transitions in Ta$_2$NiS$_5$ and the excitonic insulator candidate Ta$_2$NiSe$_5$