${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is considered a promising excitonic insulator (EI) candidate, in which the EI state is due to Coulomb interaction between electrons and holes. In ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, the role of phonons is significant since the EI transition is accompanied by an orthorhombic-to-monoclinic structural transition at $T_{\mathrm{s}}=328\mathrm{K}$. Our inelastic x-ray scattering measurements reveal that the transverse acoustic mode exhibits strong softening just above $T_{\mathrm{s}}$. Furthermore, the transverse optical mode exhibits linewidth broadening caused by strong electron–optical-phonon coupling above $T_{\mathrm{s}}$. Density functional theory calculations indicate that the coupled optical modes arise due to the vibration of Ta ions. Consequently, in the monoclinic phase, the coupled optical modes are “frozen” such that Ta and Se approach each other, forming atomic-displacement-type electric dipoles that are obviously different from a Peierls transition or Jahn–Teller distortion. The characteristic of electronic toroidal moment formation by the antiferroelectric arrangements of the electric dipoles forms the common feature between ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and $1T-\mathrm{TiS}{\mathrm{e}}_2$.

• Received 24 February 2018
• Revised 18 June 2018
• Corrected 25 January 2019

DOI:https://doi.org/10.1103/PhysRevB.98.045139