We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, tetrathiafulvalene-bromanil (TTF-BA), by ${}^{79}\mathrm{Br}\text{−}$nuclear quadrupole resonance (NQR) spectroscopy to explore cross-correlated fluctuations between spin, charge, and the lattice. A ferroelectric transition with lattice dimerization is captured by a NQR line splitting with the critical exponent $\beta$ of $0.40\pm 0.03$, close to the $\beta$ value in the three-dimensional Ising universality class, 0.33, 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 anomalously suppressed paramagnetic spin susceptibility prior to the nonmagnetic ferroelectric order as observed by Sunami et al. [Phys. Rev. Research 2, 043333 (2020)].

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• Ferroelectricity
• Lattice dynamics
• Magnetoelectric effect
• 1-dimensional spin chains
• Ferroelectrics
• Nuclear quadrupole resonance