A supramolecular crystal of 1,4-diazabicyclo[2.2.2]octane hydrochloride, [C 6 H 13 N 2 ] + Cl À $3H 2 O, exhibits extremely strong negative thermal expansion (NTE) and one-dimensional relaxor ferroelectricity, as evidenced by single-crystal X-ray and neutron diffraction, and by dielectric spectroscopy measurements.The highest ever NTE observed in solids, reflected in a volume thermal expansion coefficient of À3.86 Â 10 À4 K À1 , originates from the specific features of the honeycomb sheets formed from hydrogen bonded H 2 O molecules and Cl À anions. The NTE occurs as a result of H 2 O molecule rotations, which lead to the proton disorder and to the shortening of hydrogen bonds. The phenomenon is analogous to that in H 2 O ice, but it is distinguished by the 2D character, a much stronger magnitude, and higher temperature region. The second supramolecular pattern of the crystal, composed of the cations assembled into the strictly linear NH + /N bonded aggregates, is responsible for the relaxor-like dielectric response. The frequency-dependent and very high electric permittivity in the order of 1000, observed exclusively along the polycationic chains, results from dipolar fluctuations in the short-range polar regions, generated due to the proton transfers in the NH + /N hydrogen bonds. It has been also shown that 1,4-diazabicyclo[2.2.2]octane hydrochloride can be obtained in an anhydrous form, which is built of H-bonded ionic pairs. The loss of water changes the topology of the structure and divests the material's unprecedented properties.