Enhanced lattice fluctuations prior to a nonmagnetic ferroelectric order in an ionic spin-chain system

We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, tetrathiafulvalene-bromanil (TTF-BA), by $^{79}\mathrm{Br}\text{\ensuremath{-}}$nuclear quadrupole resonance (NQR) spectroscopy to explore cross-correlated fluctuations between spin, charge, and lattice. A ferroelectric transition with dimerization is captured a NQR line splitting critical exponent $\ensuremath{\beta}$ of $0.40\ifmmode\pm\else\textpm\fi{}0.03$, close value three-dimensional Ising universality class, 0.33, peak formation spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ at temperature ${T}_{\mathrm{c}}$, 53 K. Notably, does not obey conventional ${T}^{2}$ law expected for Raman process phonons even far above indicating emergence extraordinary fluctuations. They are very probably associated polar paraelectric paramagnetic phase TTF-BA explain anomalously suppressed spin susceptibility prior nonmagnetic order as observed Sunami et al. [Phys. Rev. Research 2, 043333 (2020)].