# Synthesis, the Reversible Isostructural Phase Transition, and the Dielectric Properties of a Functional Material Based on an Aminobenzimidazole–Iron Thiocyanate Complex

## TL;DR

A new iron thiocyanate complex with aminobenzimidazole shows a reversible structural phase transition and unique dielectric properties at low temperatures.

## Contribution

The study reports a novel functional material with a reversible isostructural phase transition and multiaxial dielectric anomalies at 240 K.

## Key findings

- Compound 1 crystallizes in the P21/n space group with a two-dimensional hydrogen-bonded network.
- The material undergoes a reversible isostructural phase transition and dielectric anomalies at ~240 K.
- Disorder–order motion of ions and framework deformation drive the phase transition and dielectric behavior.

## Abstract

By introducing disordered molecules into a crystal structure, the motion of the disordered molecules easily induces the formation of multidimensional frameworks in functional crystal materials, allowing for structural phase transitions and the realization of various dielectric properties within a certain temperature range. Here, we prepared a novel ionic complex [C7H8N3]3[Fe(NCS)6]·H2O (1) between 2-aminobenzimidazole and ferric isothiocyanate from ferric chloride hexahydrate, ammonium thiocyanate, and 2-aminobenzimidazole using the evaporation of the solvent method. The main components, the single-crystal structure, and the thermal and dielectric properties of the complex were characterized using infrared spectroscopy, elemental analysis, single-crystal X-ray diffraction, powder XRD, thermogravimetric analysis, differential scanning calorimetry, variable-temperature and variable-frequency dielectric constant tests, etc. The analysis results indicated that compound 1 belongs to the P21/n space group. Within the crystal structure, the [Fe(NCS)6]3− anion formed a two-dimensional hydrogen-bonded network with the organic cation through S···S interactions and hydrogen bonding. The disorder–order motion of the anions and cations within the crystal and the deformation of the crystal frameworks lead to a significant reversible isostructural phase transition and multiaxial dielectric anomalies of compound 1 at approximately 240 K.