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, tetrathiafulvalenebromanil (TTF-BA), by Br-79-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 +/- 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-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-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)].