Dielectric Response in Bisquaric Acid Crystal: Possible Generation of Protonic Soliton in a Quasi-One-Dimensional Hydrogen-Bonded System

We investigated the proton dynamics of crystalline bisquaric acid (H 2 BSQ) in terms of the temperature and frequency dependence of its permittivity. It was found that the dielectric response of the H 2 BSQ crystal exhibited two-step changes. With raising temperature, the dielectric permittivity increased slightly at 40 K. Whereas the temperature dependence of the permittivity at this temperature showed a large isotope effect, there was no frequency dependence of the permittivity. However, the permittivity increased largely at 290 K and showed distinct frequency dependence, although there was no-isotope effect on its characteristic temperature dependence. It was concluded that such a dielectric response of the H 2 BSQ crystal was derived from the migration of protonic soliton coupled with the polarity inversion along the quasi-one-dimensional hydrogen-bonded chain.