  Coulomb attraction between electrons and holes in a narrow-gap semiconductor
or a semimetal is predicted to lead to an elusive phase of matter dubbed
'excitonic insulator'. However, direct observation of such electronic
instability remains extremely rare. Here, we report the observation of
incipient divergence in the static excitonic susceptibility of the candidate
material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the
excitonic order parameter give rise to quasi-elastic scattering of B2g
symmetry, whose intensity grows inversely with temperature toward the Weiss
temperature of Tw ~241 K, which is arrested by a structural phase transition
driven by an acoustic phonon of the same symmetry at Tc =325 K. Concurrently, a
B2g optical phonon becomes heavily damped to the extent that its trace is
almost invisible around Tc, which manifests a strong electron-phonon coupling
that has obscured the identification of the low-temperature phase as an
excitonic insulator for more than a decade. Our result unambiguously reveals
the electronic origin of the phase transition.
