  Ta2NiSe5 is considered a promising excitonic insulator (EI) candidate with
slight phonon contributions, since it exhibits a tiny
orthorhombic-to-monoclinic structural distortion at 328 K without any
superlattice structure. Our synchrotron inelastic x-ray scattering measurements
reveal strong electron-optical-phonon coupling occurring at temperatures higher
than the transition temperature. Density functional theoretical calculations
indicate that two coupled optical modes arise due to the vibration of Ta and Se
ions. Further, the two modes are frozen such that Ta and Se approach each
other, forming atomic-displacement-type electric dipoles in the monoclinic
phase. The characteristic of electronic toroidal moment formation by the
antiferro-arrangements of electric dipoles is the universality of EI between
Ta2NiSe5 and 1T-TiSe2.
