Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

Abstract

Two phase transitions consecutively reduce the crystal symmetry with rising temperature in 1,4-diazabicyclo[2.2.2]octane hydrobromide, [C₆H₁₃N₂]⁺·Br⁻, dabcoHBr. Low-temperature prototypic hexagonal phase III, space group Pm2, Z = 1 above 458 K transforms into orientational states of orthorhombic phase II, space group Cmc2₁, Z = 4 and above 471 K to orthorhombic phase I, space group Pca2₁, Z = 8. The reverse transitions have been attributed to entropy and enthalpy contributions of the proton disordering, disproportionation of dabcoH⁺ cations into neutral dabco molecules and dabcoH²⁺₂ dications at nanodomain walls, opposite propeller versus planar dabco conformations and conversions between NH⁺⋯N and NH⁺⋯Br⁻ hydrogen bonds in dabcoHBr phase III, the NH⁺⋯N bonded relaxor material. All of the structural-disorder features in the NH⁺⋯N bonded aggregates are essential for the short-range polarization and giant dielectric response in dabcoHBr phase III. Despite the first-order type of the phase transitions, the lattice dimensions of phases I and II remain related to the prototypical relaxor phase III.