  We investigated microscopic lattice states in the donor-acceptor ionic Mott
insulator, TTF-BA, by $^{79}$Br-NQR spectroscopy to explore cross-correlated
fluctuations between spin, charge and lattice. A ferroelectric transition with
lattice dimerization is captured by a NQR line splitting with the critical
exponent $\beta$ of 0.40, as expected in the 3D Ising universality class, and a
peak formation in the spin-lattice relaxation rate $T_1^{-1}$ at the transition
temperature, $T_\mathrm{c}$, of 53 K. Notably, $T_1^{-1}$ does not obey the
conventional $T^2$ law expected for the Raman process of phonons even far above
$T_\mathrm{c}$, indicating the emergence of extraordinary lattice fluctuations.
They are very probably associated with polar fluctuations in the paraelectric
and paramagnetic phase of TTF-BA and explain the previous observation of the
anomalously suppressed paramagnetic spin susceptibility, which was conjectured
to be due to the local spin-singlet pairing prior to the nonmagnetic
ferroelectric order [K. Sunami $et$ $al$., Phys. Rev. Res. 2, 043333 (2020)].
