Proton Disorder in NH•••N Bonded [dabcoH]+I- Relaxor: New Insights into H-Disordering in a One-Dimensional H2O Ice Analogue

Protons in the NH+center dot center dot center dot N hydrogen bonds linking the cations into strictly linear aggregates in the crystal of 1.4dit1ztibicyclo[2.2.2loctane hydroiodide ([C6H13N2](+)center dot I- dabcoH1) remain disordered. and the average crystal symmetry (space group P (6) over barm(2)) remains unchanged, clown to 1.5 K, as evidenced by single-crystal neutron-diffraction, but against the expectation based on the third law of thermodynamics. This proton disorder reveals the role of the thermal-activation and proton-tunneling processes for the polarizability of hydrogen bonds and for the formation of polar nanodomains, resulting in the unique property or an anisotropic giant dielectric response in the chemically homogeneous compound. The dielectric constant exceeding several thousand in the direction of the crystal [z] axis between 150 and 300 K can be explained by the fluctuations of nanodomains, whereas the dielectric constant of about 100 at 10 K can be due to the residual contribution of tunneling protons in hydrogen bonds. The proton transfers disproportionate the dabco mcmocations into dications and neutral molecules and generate high crystal-held fluctuations. The consideration of possible crystal-symmetry changes shows that ferroelectric proton ordering would lead to the lowered symmetry of space group P3m1 within the hexagonal system, and antiferroelectric proton ordering would require a transformation to an orthorhombic phase; however, the proton-site correlations are too weak for these transformations to take place, and the resultant structure remains disordered, The crystal symmetry and the lack of Fowler Bernal rules distinguish dabcoH1 from the H2O ice-I-h, and are essential for the dielectric properties of this substance.