Effect of halide solid solution on the structure, phase transition behaviour and dielectric properties of dabcoH+ chains

Abstract

1,4-Diazabicyclo[2.2.2]octane (dabco) forms one-dimensional chains through intermolecular N–H⁺⋯N hydrogen bonds and exhibits ferroelectricity originating from proton transfer. The size of the polarized domain differs depending on the size, symmetry, and polarizability of the counter anions. Herein, two binary halide solid solutions of dabcoHBr mixed with dabcoHCl (ClBr) and dabcoHI (BrI) were prepared, and the temperature dependence of the single crystal structure and the phase transition behaviour were investigated. The solid solution of ClBr contained 3% Cl⁻ in dabcoHBr; thus, the lattice length, space group, and volume were similar to those of dabcHBr. The phase transition temperature from the hydrogen-bonded structure to the ion-pair structure decreased from 458 to 440 K upon mixing with Cl⁻. In contrast, solid solution BrI was formed with all proportional solid solutions, and the crystallographic parameters characterised by single-crystal X-ray diffraction (XRD) analysis obeyed Vegard's law. Phase transition behaviours were investigated for single crystals of Br : I = 8 : 2 (Br_0.80I_0.20) via temperature-variable single crystal XRD analysis. A new structural range was observed between 251–373 K. Within this range, the intermolecular N–H⁺⋯N hydrogen bonds were weakened. This was also a ferroelectric phase similar to that of the original dabcoHBr. In the high-temperature range, a phase transition from the hydrogen-bonded phase to the ion-pair phase was observed at 435 K, which was lower than the 458 K of dabcoHBr.