Strong negative thermal expansion and relaxor ferroelectricity driven by supramolecular patterns

Abstract

A supramolecular crystal of 1,4-diazabicyclo[2.2.2]octane hydrochloride, [C₆H₁₃N₂]⁺Cl⁻·3H₂O, 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 −3.86 × 10⁻⁴ K⁻¹, originates from the specific features of the honeycomb sheets formed from hydrogen bonded H₂O molecules and Cl⁻ anions. The NTE occurs as a result of H₂O molecule rotations, which lead to the proton disorder and to the shortening of hydrogen bonds. The phenomenon is analogous to that in H₂O 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.