Halogen Substitution in Inorganics for the Regulation of Phase Transition Temperature in Zero-Dimensional Organic-Inorganic Hybrid Metal Halide Dielectric Switching Materials

Abstract

Zero-dimensional (0D) organic-inorganic hybrid metal halide material (OIMH) [C₃H₆NH₂]₂CoBr₄ (C₃H₆NH₂ = azetidine cation) was successfully synthesized and carried out a series of characterizations. A comparison with [C₃H₆NH₂]₂CoCl₄ was then conducted. It was found that after the anionic halogen substitution, the dielectric switching properties remained essentially consistent, and the main reason for the phase transition was the cationic order-disorder motion. However, the phase transition temperature (T_C) of [C₃H₆NH₂]₂CoBr₄ is 364.5 K, which is 16.8 K higher than that of [C₃H₆NH₂]₂CoCl₄. Calculations revealed that the void occupancy between cations and anions decreased from 32.49% in [C₃H₆NH₂]₂CoCl₄ to 29.77% in [C₃H₆NH₂]₂CoBr₄, and due to the reduction in porosity, the temperature required for cationic order-disorder motion is therefore higher. Additionally, the analysis of the Hirshfeld surface and two-dimensional fingerprints revealed that the proportion of intermolecular hydrogen bond interactions in [C₃H₆NH₂]₂CoBr₄ (47.1%) is higher than in [C₃H₆NH₂]₂CoCl₄ (45.75%), and there is also an enhancement in the non-covalent interaction energies between cations and anions (the non-covalent interaction energies between cations and anions in [C₃H₆NH₂]₂CoCl₄ are –6.9 and –8.7 kJ/mol, while in [C₃H₆NH₂]₂CoBr₄ they are –7.6 and –20.9 kJ/mol, which is also the reasons for the increase in T_C. Furthermore, we observed that as the volume of anions in the inorganic framework increased, the band gap value also showed a decreasing trend, from 4 eV for [C₃H₆NH₂]₂CoBr₄ to 3.678 eV for [C₃H₆NH₂]₂CoCl₄.